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THE  MACMILLAN  CO.  OF  CANADA,  Lm 

TORONTO 


GENETICS 

AN  INTRODUCTION  TO  THE  STUDY 
OF  HEREDITY 


BY 

HERBERT  EUGENE  WALTER 

PROFESSOR   OF    BIOLOGY,    BROWN   UNIVERSITY 


WITH  92  FIGURES  AND  DIAGRAMS 


REVISED    EDITION 


THE  MACMILLAN  COMPANY 
1924 

All  rights  reserved 


PREFACE  TO  THE  REVISED  EDITION 

NEARLY  ten  years  have  passed  since  this  book  first 
appeared.  The  biological  Rip  van  Winkle  of  today 
who,  awaking  after  a  decade  of  somnolence,  gazes  again 
upon  the  genetic  village  of  Falling  Waters,  will  indeed 
need  to  rub  his  astonished  eyes  at  the  changed  scene 
that  now  spreads  out  before  him.  Many  old  familiar 
landmarks,  such  as  "unit  characters"  and  "dominance," 
show  signs  of  dilapidation,  while  strange  children, 
shouting  a  medley  of  outlandish  words,  "linkage," 
"tetraploidy,"  and  "non-disjunction,"  for  example,  are 
playing  new  games  on  the  village  green. 

Although  the  remarkable  advances  in  this  field  of 
science  are  well  treated  in  considerable  detail  by  several 
recent  text-books,  notably  those  of  Castle,  Morgan, 
Conklin,  and  Babcock  and  Clausen,  perhaps  there  still 
remains  the  original  need  for  a  more  elementary  pre- 
sentation of  the  salient  points  of  genetics,  not  only  for 
the  interested  but  confused  layman,  but  also  for  the 
initiation  of  the  prospective  student  who  is  attracted  to 
the  study  of  heredity. 

To  perform  this  service  is  the  ambitious  object  of 
the  present  revision. 

Three  new  chapters,  XI,  XII  and  XIII,  have  been 
added  and  the  whole  book  has  been  thoroughly  worked 
over  and  rearranged.  Chapter  XIII  upon  Sex  De- 
termination has  practically  been  written  by  Professor 
S.  I.  Kornhauser  of  Denison  University  and  the  entire 

vii 


viii    PREFACE  TO  THE  REVISED  EDITION 

manuscript  critically  read  by  Dr.  J.  W.  Wilson  of 
Brown  University. 

There  are  thirty-four  new  figures  and  diagrams 
which  are  either  original  or  copied  from  acknowledged 
sources.  Mr.  C.  J.  Fish  made  the  drawings  for  figures 
22  and  32.  The  proof  was  read  by  my  wife  and  by  my 
niece,  Miss  Dorothy  Walter. 

I  wish  to  acknowledge  the  help  I  have  received  from 
all  of  these  sources  as  well  as  from  many  unnamed 
friends  who  have  given  valuable  suggestions. 

H.  E.  W. 

LA  JOLUA,  CALIF., 
March  18, 


PREFACE  TO  THE  FIRST  EDITION 

THE  following  pages  had  their  origin  in  a  course 
of  lectures  upon  Heredity,  given  at  Brown  University 
during  the  winter  of  1911-1912,  which  were  amplified 
and  repeated  in  part  the  following  summer  at  Cold 
Spring  Harbor,  Long  Island,  before  the  biological 
summer  school  of  the  Brooklyn  Institute  of  Arts  and 
Sciences. 

An  attempt  has  been  made  to  summarize  for  the 
intelligent,  but  uninitiated,  reader  some  of  the  more 
recent  phases  of  the  questions  of  heredity  which  are  at 
present  agitating  the  biological  world.  It  is  hoped 
that  this  summary  will  not  only  be  of  interest  to  the 
general  reader,  but  that  it  will  also  be  of  service  in 
college  courses  dealing  with  evolution  and  heredity. 

The  subject  of  heredity  concerns  every  one,  but 
many  of  those  who  wish  to  become  better  informed 
regarding  it  are  either  too  busily  engaged  or  lack  the 
opportunity  to  study  the  matter  out  for  themselves. 
The  recent  literature  in  this  field  is  already  very  large, 
with  every  indication  that  much  more  is  about  to 
follow,  which  is  a  further  discouragement  to  non- 
technical readers. 

It  may  not  be  a  thankless  task,  therefore,  out  of 
the  jargon  of  many  tongues  to  raise  a  single  voice 
which  shall  attempt  to  tell  the  tale  of  heredity.  There 
may  be  a  certain  advantage  in  having  as  spokesman 
one  who  is  not  at  present  immersed  in  the  arduous 

ix 


x       PREFACE   TO   THE   FIRST   EDITION 

technical  investigations  that  are  making  the  tale  worth 
telling.  The  difficulties  in  understanding  this  compli- 
cated subject  may  possibly  be  realized  better  by  one 
who  is  himself  still  struggling  with  them,  than  by  the 
seasoned  expert  who  has  long  since  forgotten  that  such 
difficulties  exist. 

Among  others  I  am  particularly  indebted  to  Dr. 
C.  B.  Davenport  for  many  helpful  suggestions,  to 
my  colleague,  Professor  A.  D.  Mead,  for  reading  the 
manuscript  critically,  to  Dr.  S.  I.  Kornhauser  who 
gave  valuable  aid  in  connection  'with  the  chapter 
on  the  Determination  of  Sex,  and  to  my  wife  for 
assistance  in  final  preparation  for  the  press. 

I  wish  to  thank  Professor  H.  S.  Jennings  and  Dr. 
H.  H.  Goddard,  who  have  given  generous  permission 
to  copy  certain  diagrams,  as  well  as  The  Outlook 
Company  and  The  Macmillan  Company  for  the  use 
of  figures  24  and  66,  respectively. 

The  fact  that  not  all  the  suggestions  which  were  at 
various  times  offered  by  my  kindly  critics  have  been  in- 
corporated in  the  text,  absolves  them  from  responsibility 
for  whatever  remains. 

H.  E.  W. 

PROVIDENCE,  R.  I.f 
September,  1912 


CONTENTS 

CHAPTER  PAGE 

I.    INTRODUCTION 1 

1.  The  idea  of  species 1 

2.  The   triangle   of   life 3 

3.  A  definition  of  heredity 6  *-" 

4.  The  maintenance  of  life 8 

5.  Somatoplasm  and  germplasm 12  \) 

II.  VARIATION 17 

1.  The  most  invariable  thing  in  nature 17 

2.  The  universality  of  variation 18 

3.  The  kinds  of  variation  with  respect  to  their: — 

a.  Nature 19 

b.  Duplication 20 

c.  Utility 20 

d.  Direction  in  evolution 21 

e.  Source .  21 

f.  Normality 22 

g.  Degree  of  continuity 22 

h.  Character 22 

i.  Relation  to  an  average  standard  ....  23 

j.  Heritability 23  <— 

4.  Methods  of  studying  variation 

5.  Biometry        23 

6.  Fluctuating   variation 24 

7.  The  interpretation  of  variation  curves  ....  28 

a.  Relative  variability 28 

b.  Bimodal  curves 29 

c.  Skew  curves 31 

8.  Graduated  and  integral  variations 33  ^ 

9.  The  causes  of  variation 34    / 

a.  Darwin's  attitude 34 * 

b.  Lamarck's    attitude 34  ^ 

c.  Weismann's  attitude 35  tx 

d.  Bateson's  attitude 37 

III.  HERITABLE  DIFFERENCES 

1.  The  mutation  idea 38  ^ 

2.  A  summary  of  the  mutation  theory 41 

xi 


xii  CONTENTS 

CHAPTER 

3.  Lamarck's  evening  primrose 42 

4.  Plant  mutations  found  in  nature 47 

5.  Some  mutations  among  animals 48 

6.  Kinds  of  mutation 51 

7.  The  origin  of  mutations 53 

8.  When  mutations  occur 55 

9.  Possible  causes  of  mutation 57 

IV.    THE  INHERITANCE  OF  ACQUIRED  CHARACTERS      ...  62 

1.  Summary  of  preceding  chapters 62 

2.  The  bearing  of  this  chapter  upon  genetics  .     .  63 

3.  The  importance  of  the  question 63 

4.  A  historical  sketch  of  opinion 64 

5.  Confusion  in  definitions 66 

6.  Weismann's  conception  of  acquired  characters  .  67' 

7.  The    distinction    between    germinal    and    somatic 

characters 67 

8.  What  variations  reappear? 69 

9.  How  may  germplasm  acquire  new  characters?     .  69 

10.  Weismann's  reasons  for  doubting  the  inheritance 

of  acquired  characters 71 

a.  No  known  mechanism  for  impressing  germ- 

plasm  with  somatic  characters     ...  71 

b.  Evidence   for  the  inheritance   of   acquired 

characters  inconclusive 74 

a.  Mutilation 75 

b.  Environmental  effects 76 

c.  The  effects  of  use  or  disuse  ....  78 

d.  Transmission  of  disease 80 

e.  Immunity  and  the  effect  of  drugs  .     .  82 

f.  Prenatal  influences 83 

c.  The  germplasm  theory  sufficient  to  account 

for  the  facts  of  heredity 84 

11.  The  comparative  independence  of  germ  and  soma  85 

12.  Acquired  characters  in  the  protozoa   ....  86 

13.  The  opposition  to  Weismann 88 

14.  Various  results  upon  the  offspring  of  parental 

acquisitions 90 

15.  Conclusion 92 

V.   MENDELISM        93 

1.  Methods  of  studying  heredity 93 

2.  The  melting  pot  of  cross-breeding 93 

3.  Johann  Gregor  Mendel 95 

4.  Mendel's  experiments  on  garden  peas  ....  97 

5.  Some  further  instances  of  Mendel's  law  .     .     .  101 

6.  The  cardinal  principle  of  segregation  ....  103 


CONTENTS  xiii 

CHAPTER 


7.  Definitions      ............  104 

8.  The  identification  of  a  heterozygote       .     .     .      .  105 

9.  The  presence  or  absence  hypothesis    ....  106 

10.  Dihybrids        ............  107 

11.  The  case  of  the  trihybrid  ........  114 

12.  Summary        ...........     .117 

13.  The  practical  application  ........  119 

14.  Conclusion      ............  120 


VI.   THE  PURE  LINE  AND  SELECTION 


1.  Galton's  law  of  regression  ........  121 

2.  The  idea  of  the  pure  line    .......  122 

3.  Johannsen's  nineteen   beans    ........  124 

4.  The  distinction  between  a  population  and  a  pure 

line    ..............  127 

5.  Cases  similar  to  Johannsen's  pure  lines   .     .     .  132 

6.  Selection  within  the  pure  line   ......  133 

a.  Vilmorin's  wheat  .........  135 

b.  Clones     ............  135 

c.  Parthenogenetic  progeny      ......  141 

d.  Homozygous  crosses  ........  142 

a.  Tower's  potato  beetles      .....  143 

6.  Drosophila  bristles  .......  143 

c.  Pearl's  200-egg  hen     ......  145 

d.  Castle's  hooded  rats    ......  145 

7.  Conclusion      ............  146 

VII.   THE  FACTOR  HYPOTHESIS     .........  148 

1.  The  hereditary  unit      .........  148 

2.  Different  kinds  of  genes  ........  149 

3.  Complementary  genes   .........  151 

4.  Supplementary  genes    .........  154 

a.  Castle's  agouti  guinea-pigs  ......  154 

b.  Cuenot's  spotted  mice     .......  155 

c.  Miss  Durham's  intensified  mice       ....  156 

d.  Castle's  brown-eyed  yellow  guinea-pigs  .     .  157 

e.  Rabbit  phenotypes     ........  159 

f.  The  kinds  of  gray  rabbits  ......  162 

5.  Lethal  genes  .......     .....  162 

6.  Modifying  genes  and  selection   .»..».  166 

VIII.   BLENDING  INHERITANCE  ..........  168 

1.  The  relative  significance  of  dominance  and  seg- 

regation   ............  168 

2.  Imperfect  dominance   .....     ....  168 

3.  Delayed  dominance  ..........  170 


xvi  CONTENTS 

CHAPTER  FAGB 

XIV.   THE  APPLICATION  TO  MAN 296 

1.  The  application  of  genetics  to  man    .     .     .      .296 

2.  Modifying  factors  in  the  case  of  man  .     .     .     .297 

3.  Experiments  in  human  heredity 299 

a.  The  Jukes 299 

b.  The  descendants  of  Jonathan  Edwards  .     .  301 

c.  The  Kallikak   family 302 

4.  Moral  and  mental  characters  behave  like  physical 

ones 303 

5.  The  character  of  human  traits 304 

6.  Hereditary  defects 306 

7.  The  control  of  defects 309 

XV.    HUMAN   CONSERVATION 314 

1.  How  mankind  may  be  improved 314 

2.  Human  assets  and  liabilities 315 

3.  More  facts  needed 317 

4.  Further  application  of  what  we  know,  necessary  319 

5.  The  restriction  of  undesirable  germplasm  .      .      .  320 

a.  Control  of  immigration 322 

b.  More  discriminating  marriage  laws   .      .      .  324 

c.  An  educated  sentiment 325 

d.  Segregation  of  defectives 327 

e.  Drastic  measures 329 

6.  The  conservation  of  desirable  germplasm  .     .     .  330 

a.  By  enlarging  individual  opportunity  .      .     .  331 

b.  By  preventing  germinal  waste 331 

a.  Preventable  death 331 

6.  Social  hindrances 

c.  By  subsidizing  the  fit 333 

7.  Who  shall  sit  in  judgment? 334 

8.  Eugenics  not  "bluegenics" 335 

9.  The   moral 336 

BIBLIOGRAPHY 337 

INDEX  941 


GENETICS 


GENETICS 

CHAPTER  I 

INTRODUCTION 

1.  THE  IDEA  OF  SPECIES 

THE  doctors  have  always  disagreed  regarding  a 
definition  of  species.  What  determines  the  exclusive 
boundaries  that  shall  isolate  from  their  fellows  any 
particular  group  of  animals  or  plants  has  long  been 
a  mooted  question,  and  still  remains  so. 

The  Linnsean  concept  of  a  species  was  that  of  an 
exclusive  caste  of  individuals,  inflexibly  demarked, 
over  whose  high  barriers  no  nondescript  tramps 
would  dare  attempt  to  climb.  When  an  entomolo- 
gist of  the  old  Linnsean  school  encountered  an  insect 
which  did  not  conform  to  the  morphological  tradi- 
tions of  its  fellows,  the  frequent  fate  of  such  a  non- 
conformist was  to  perish  under  the  boot-heel  rather 
than  to  find  sanctuary  in  the  cabinet  of  the  preserved. 
Since  it  was  an  exception,  and  a  violator  of  the  divine 
law  of  the  fixity  of  species,  it  deserved  to  be  anni- 
hilated! Those  were  hard  days  both  for  heretics  and 
for  variations. 

The  method  of  the  older  school  of  systematists 

1 


GENETICS 


may  be  described  as  one  which  emphasized  differences 
and  put  up  barriers  that  should  keep  the  unlike  apart, 
at  the  same  time  allowing  only  "birds  of  a  feather" 
to  flock  together.  It  was  a  brave  and  successful 
attempt  to  bring  order  out  of  chaos  by  classifying 
the  living  world,  and  it  served  its  purpose  well  until 
Darwin's  idea  of  half  a  century  ago,  that  the  origin 
of  all  species  is  from  preceding  species,  put  an  en- 
tirely new  face  upon  the  whole  matter.  Organisms 
of  different  species  were  found  to  be  related  to  one 
another,  and  even  man  could  no  longer  escape  ac- 
knowledging his  poor  animal  relations.  As  a  conse- 
quence, likenesses  rather  than  differences  thereafter 
claimed  the  most  attention. 

During  the  reconstruction  of  pihylogenetic  trees, 
which  seized  the  imagination  and  became  the  prin- 
cipal business  of  post-Darwinian  biologists,  "connect- 
ing links,"  that  is,  the  crotched  sticks  in  the  woodpile 
of  organisms,  which  had  hitherto  been  largely  dis- 
carded, were  most  eagerly  sought  after.  It  was  just 
these  scraggly  sticks,  that  were  neither  trunk  nor 
limb-wood  but  combinations  of  both,  which  told  the 
story  of  continuity  and  were  indispensable  in  building 
up  a  reunited  whole. 

As  the  analysis  of  the  living  world  gradually  came 
to  shift  from  species  to  individuals,  it  was  shown  that 
individuals  may  be  regarded  simply  as  aggregates 
of  imit  characters  which  may  combine  so  variously 
that  it  becomes  more  and  more  difficult  to  maintain 
constant  barriers  of  any  kind  between  the  groups  of 
individuals  arbitrarily  called  "species." 


INTRODUCTION  3 

2.  THE  TRIANGLE  OF  LIFE 

Accordingly  within  a  generation  the  center  of  bio- 
logical interest  gradually  swung  from  the  origin  of 
species  to  the  origin  of  the  individual.  The  nineteenth 
century  was  Darwin's  century.  His  monumental  work 
"On  the  Origin  of  Species  by  Means  of  Natural 
Selection,"  which  appeared  in  1859,  not  only  dominated 
the  biological  sciences  but  also  influenced  profoundly 
many  other  realms  of  thought,  particularly  those  of 
philosophy  and  theology. 

Now,  in  the  first  decades  of  the  twentieth  century, 
a  particular  emphasis  is  being  laid  upon  the  study  of 
heredity.  The  interpretation  of  investigations  along 
this  line  of  research  has  been  made  possible  through 
the  cumulative  discoveries  of  many  things  that  were 
not  known  in  Darwin's  day.  Trained  students,  pa- 
tiently and  persistently  bending  over  improved  micro- 
scopes, have  untangled  the  mysteries  of  the  cell,  while  an 
increasing  host  of  investigators,  inspired  by  the 
Austrian  monk  Mendel,  have  industriously  devoted  their 
energies  to  breeding  animals  and  plants  with  an  insight 
denied  to  breeders  of  preceding  centuries. 

The  study  of  the  origin  of  the  individual,  which 
has  grown  out  of  the  more  general  consideration  of 
the  origin  of  species,  forms  the  subject-matter  of 
heredity,  or,  to  use  the  more  definitive  word  of  Bate- 
son,  of  genetics. 

(It  is  not  with  the  individual  as  a  whole  that 
genetics  is  chiefly  concerned,  but  rather  with  char- 
acteristics of  the  individual. 


4  GENETICS 

Three  factors  acting  together  determine  the  char- 
acteristics of  an  individual,  namely,  environment,  re- 
sponse, and  heritage,  as  expressed  diagrammatically 
in  Figure  1.  It  may  be  said  that  an  individual  is  the 
result  of  the  interaction  of  these  three  factors  since  he 
may  be  modified  by  changing  any  one  of  them.  Although 
no  one  factor  can  possibly  be  omitted,  the  student  of 
genetics  places  the  emphasis  upon  heritage  as  the 


Heritage 
FIG.  1. — The  triangle  of  life. 

factor  of  greatest  importance.  Heritage,  or  "blood," 
expresses  the  innate  equipment  of  the  individual.  JTt  Js 
what  he  actually  is  even  before  birth.  It  is  his  nature. 
It  is  what  determines  whether  he  shall  be  a  beast  or  a 
man.  Consequently  in  the  diagram  (Fig.  1),  the  tri- 
angle of  life  is  represented  as  resting  solidly  upon 
the  side  marked  "heritage"  for  its  foundation. 

Environment  and  response,  although  indispensable, 
are  both  factors  which  are  subsequent  and  secondary. 
Environment  is  what  the  individual  has,  for  example, 
housing,  food,  friends  and  enemies  or  surrounding  aids 
which  may  help  him  and  obstacles  which  he  must 


INTRODUCTION  5 

overcome.  It  is  the  particular  world  into  which  he 
comes,  the  measure  of  opportunity  given  to  his  par- 
ticular heritage. 

Response,  on  the  other  hand,  represents  what  the 
individual  does  with  his  heritage  and  environment.  It 
is  what  may  be  described  as  the  training  or  educa- 
tional factor.  Lacking  a  suitable  environment  a  good 
heritage  may  come  to  naught  like  good  seed  sown  upon 
stony  ground,  but  it  is  nevertheless  true  that  the  best 
environment  cannot  make  up  for  defective  heritage 
or  develop  wheat  from  tares. 

The  absence  of  sufficient  response  even  when  the 
environment  is  suitable  and  the  endowment  of  inherit- 
ance is  ample  will  result  in  an  individual  who  falls  short 
of  his  possibilities,  while  no  amount  of  response  or  edu- 
cation can  develop  a  man  out  of  the  heritage  of  a  beast. 
Consequently  the  biologist  holds  that,  although  what 
an  individual  has  and  does  is  unquestionably  of  great 
importance,  particularly  to  the  individual  himself, 
what  he  is,  is  in  the  long  run  far  more  important. 
Improved  environment  and  training  may  better  the 
generation  already  born.  Improved  blood  will  better 
every  generation  to  come.  The  "triangle  of  life,"  when 
applied  to  man,  shows  that  there  are  theoretically  at 
least  twenty-seven  possible  kinds  of  human  beings  as 
shown  in  Figure  £.  Climbing  up  this  "scale  of  success'* 
is  what  makes  life  worth  living.  It  is  illuminating 
for  any  one  to  determine  judiciously  where  he  him- 
self stands  at  present  or  to  assign  places  mentally  to 
various  other  people,  historical  and  contemporary,  in 
this  scale. 

The  left-hand  factor  does  not  change  throughout 


6 


GENETICS 


Ill 

&  fl  <£ 

n  w  tf 

1. 

AAA 

2. 

AAB 

3. 

AAC 

4. 

ABA 

5. 

ABB 

6. 

ABC 

7. 

ACA 

8. 

ACB 

9. 

ACC 

10. 

BAA 

11. 

BAB 

12. 

BAG 

13. 

BBA 

14. 

BBB 

15. 

BBC 

16. 

BCA 

17. 

BCB 

18. 

BCC 

19. 

CAA 

20. 

CAB 

21. 

CAC 

22. 

CBA 

23. 

CBB 

24. 

CBC 

25. 

CCA 

26. 

CCB 

27. 

CCC 

FIG.  2.— The  Scale  of  Success. 
A  stands  for  high  grade; 

B,  for  mediocrity; 

C,  for  low  grade. 


life  but  the  other  two  may. 
The  sociologist  and  the 
philanthropist  are  imme- 
diately concerned  with  the 
middle  column;  the  edu- 
cator and  particularly  the 
parent  with  the  right-hand 
column;  while  the  biologist 
puts  faith  in  the  left-hand 
column  of  heritage.  For 
example,  a  child  born  ACC 
is  more  apt  to  reach  the 
top  than  one  born  CCC.  In 
selecting  a  mate  it  would  be 
far  wiser  to  marry  ACC 
than  CAA,  since  "blood 
wiU  tell." 

What,  then,  is  this 
"blood"  or  heritage?  Ex- 
actly what  is  meant  by 
heredity  ? 

3.  A  DEFINITION  OP 
HEREDITY 

The  terms  heredity  and 
inheritance  come  to  us  from 
legal  practice.  We  "in- 
herit" the  old  homestead  or 
our  grandfather's  clock. 
Moreover,  as  "heirs  to  all 
the  ages"  our  heredity  in- 


INTRODUCTION  7 

eludes  everything  that  goes  to  make  up  civilization, 
such  as  the  arts,  sciences,  literature  and  traditions. 
With  this  kind  of  heredity  we  are  not  here  concerned, 
for  this  is  not  what  is  meant  by  biological  heredity. 

Professor  Castle,  in  his  book  on  "Heredity  in  Rela- 
tion to  Evolution  and  Animal  Breeding,"  has  defined 
heredity  as  "organic  resemblance  based  on  descent."  ' 
The  son  resembles  his  father  because  he  is  a  "chip  off 
the  old  block."  It  would  be  still  nearer  the  truth  to  say 
that  the  son  resembles  his  father  because  they  are 
both  chips  from  the  same  block,  since  the  actual  char- 
acters of  parents  are  never  transmitted  to  their  off- 
spring in  the  same  way  that  real  estate  or  personal 
property  is  passed  on  from  one  generation  to  another. 
When  the  son  is  said  to  have  his  father's  hair  and  his 
mother's  complexion  it  does  not  mean  that  paternal 
baldness  and  a  vanishing  maternal  complexion  are 
the  inevitable  consequences. 

Biological  inheritance  is  more  comparable  to  the 
handing  down  from  father  to  son  of  some  valuable 
patent  right  or  manufacturing  plant  by  means  of 
which  the  son,  in  due  course  of  time,  may  develop  an 
independent  fortune  of  his  own,  resembling  in  char- 
acter and  extent  the  parental  fortune  similarly  de- 
rived although  not  identical  with  it. 

So  it  comes  about  that  "organic  resemblance"  be- 
tween father  and  son,  as  well  as  that  which  often 
appears  between  nephew  and  uncle  or  even  more  re- 
mote relatives,  is  due  not  to  a  direct  entail  of  the 
characteristics  in  question,  but  to  the  fact  that  the 
characteristics  are  "based  on  descent"  from  a  common 


8  GENETICS 

source.  In  other  words,  an  "hereditary  character"  of 
any  kind  is  not  an  entity  or  unit  which  is  handed 
down  from  generation  to  generation,  but  is  rather  a 
method  of  reaction  of  the  organism  to  the  constellation 
of  external  environmental  factors  under  which  the 
organism  lives. 

To  unravel  the  golden  threads  of  inheritance  which 
have  bound  us  all  together  in  the  past,  as  well  as  to 
learn  how  to  weave  upon  the  loom  of  the  future,  not 
only  those  old  patterns  in  plants  and  animals  and  men 
which  have  already  proven  worth  while,  but  also  to 
create  new  organic  designs  of  an  excellence  hitherto 
impossible  or  undreamed  of,  is  the  inspiring  task  before 
the  geneticist  to-day. 

4.  THE  MAINTENANCE  OF  LIFE 

So  far  as  we  know,  every  living  thing  on  the  earth 
to-day  has  arisen  from  some  preceding  form  of  life. 

How  the  first  spark  of  life  began  will  probably 
always  be  a  matter  of  pure  speculation.  Whether 
the  beginnings  of  what  is  called  life  came  through 
space  from  other  worlds  on  meteoric  wings,  as  Lord 
Kelvin  has  suggested;  whether  it  was  spontaneously 
generated  on  the  spot  out  of  lifeless  components; 
or  whether  life  itself  was  the  original  condition  of 
matter,  and  the  one  thing  that  must  be  explained  is 
not  the  origin  of  life  but  of  the  non-living,  no  one 
can  say.  Leaving  aside  the  first  speculation  as  un- 
tenable and  the  third  as  irrational,  since  it  jars  so 
sadly  with  what  astronomers  tell  us  of  the  probable 


INTRODUCTION  9 

evolution  of  worlds,  the  theory  of  spontaneous  genera- 
tion seems  to  be  the  last  resort  to  which  to  turn. 

In  prescientific  days  this  idea  of  spontaneous  genera- 
tion presented  no  great  difficulties  to  our  imaginative 
and  credulous  ancestors.  John  Milton,  with  the  assur- 
ance of  an  eye-witness,  thus  described  the  inorganic 
origin  of  a  lion: — 

"The  grassy  clods  now  calved;  now  half  appears 
The  tawny  liorr,  pawing  to  get  free 
His  hinder  parts  —  then  springs  as  broke  from  bonds, 
And  rampant  shakes  his  brindled  mane." 

("Paradise  Lost/'  Book  VII,  line  543.) 

Ovid  also  in  his  "Metamorphoses,"  not  to  mention  a 
more  familiar  instance  of  special  creation,  easily  suc- 
ceeded in  creating  mankind  from  the  humble  stones 
tossed  by  the  juggling  hands  of  Deucalion  and  Pyrrha. 

Although  under  former  conditions  on  the  earth 
it  might  have  been  possible  for  life  to  have  originated 
spontaneously,  and  although  it  may  yet  be  possible 
to  produce  life  from  inorganic  materials  in  the  labora- 
tory or  elsewhere,  the  exhaustive  work  of  Pasteur, 
Tyndall  and  others  effectually  demonstrated  a  genera- 
tion ago  that  to-day  living  matter  always  arises  from 
preceding  living  matter  and  this  conclusion  is  gener- 
ally accepted  as  an  axiom  in  genetics. 

There  are  various  methods  of  producing  more  life, 
given  a  nest-egg  of  living  substance  with  which  to 
start.  Any  organism,  whether  plant  or  animal,  is 
continually  transforming  inorganic  and  dead  material 
into  living  tissue.  Through  the  process  of  repair,  for 
example,  an  injury  to  a  form  as  highly  developed 


10  GENETICS 

even  as  man  is  frequently  made  good,  if  it  is  not  too 
extensive  and  does  not  involve  too  highly  specialized 
tissues,  as,  for  example,  in  the  case  of  a  skin  wound. 

When  the  intake  of  non-living  material  is  in  excess 
of  the  outgo,  growth  results,  with  the  consequence 
that  more  living  substance  is  built  up  than  existed 
before.  Thus  a  fragment  of  a  living  sponge  or  a 
piece  of  a  begonia  leaf  is  each  sufficient  to  restore  a 
duplicate  of  the  original  organism. 

A  process  similar  to  the  repair  of  the  begonia  leaf 
is  that  employed  so  effectively  in  the  great  groups  of 
the  one-celled  animals  and  plants,  the  Protozoa  and 
Protophyta,  by  means  of  which  their  numbers  are 
maintained.  These  one-celled  organisms  usually  multi- 
ply by  fission,  that  is,  by  division  into  halves,  and 
each  half  grows  to  the  size  of  the  parent  organism 
from  which  it  sprang.  When  two  daughter  protozoans 
are  thus  formed,  they  are  essentially  orphans  because 
they  have  no  parents,  alive  or  dead.  The  parental 
substance  in  such  a  process,  along  with  the  regulating 
power  necessary  to  reorganization,  goes  over  bodily 
into  the  next  generation  in  the  formation  of  the 
daughter-cells,  leaving  usually  no  remains  whatever 
behind.  In  primitive  forms  of  this  description,  con- 
tinuous life  is  the  natural  order,  and  death,  when  it 
does  occur,  is,  as  Weismann  has  pointed  out,  acci- 
dental and  quite  outside  the  plan  of  nature. 

In  these  cases,  it  is  easy  to  see  the  reason  for  "or- 
ganic resemblance"  between  successive  generations. 
Parent  and  offspring  are  successive  manifestations 
of  the  same  thmg,  just  as  the  begonia  plant,  restored 


INTRODUCTION  11 

from  a  fragment  of  a  begonia  leaf,  is  simply  an  ex- 
tension of  the  original  plant. 

Many  modifications  of  the  process  of  multiplication 
by  fission  occur,  all  of  them,  however,  agreeing  in  the 
fundamental  principle  that  the  progeny  resemble  the 
parents  because  they  are  pieces  of  the  parents. 

Thus  the  "greening"  apple  maintains  its  individu- 
ality although  coming  from  thousands  of  different  trees, 
because  all  of  these  trees  through  the  asexual  process 
of  grafting  are  continuations  of  the  one  original 
Rhode  Island  greening  tree  grown  by  Dr.  Solomon 
Drowne  in  the  town  of  Foster,  nearly  a  century  ago. 
Western  navel  oranges  all  come,  directly  or  indirectly, 
from  parts  of  one  tree  found  near  Bahia  in  Brazil. 

Again,  certain  fresh-water  sponges  and  bryozoans, 
quite  unlike  most  of  their  marine  relatives,  keep  a 
foothold  from  year  to  year  within  their  particular 
shallow  fresh-water  habitats  by  isolating  well  pro- 
tected fragments  of  themselves  in  the  form  of  geimrndes 
and  statoblasts.  These  structures  may  drop  to  the 
muddy  bottom  and  live  in  a  dormant  condition  through- 
out the  icy  winter  when  it  would  not  be  possible  for 
the  entire  organism  to  survive  near  the  surface. 

In  order  to  meet  the  conditions  imposed  by  winter, 
however,  these  fragments  have  become  so  modified  as 
temporarily  to  lose  their  likeness  to  the  parent  genera- 
tion, although  readily  regaining  that  likeness  when 
springtime  brings  the  opportunity.  The  unity  of 
two  succeeding  generations,  notwithstanding  that  it 
may  be  interrupted  by  the  temporary  interposition  of 
something  apparently  different  in  the  form  of  gemmules 


12  GENETICS 

or  statoblasts,  is  thus  essentially  maintained.  The 
bryozoan  colonies  of  two  successive  seasons  in  a  fresh- 
water pond  may  be  regarded  as  parts  of  the  same 
identical  colony,  since  they  present  an  "organic  resem- 
blance based  on  descent,"  although  the  sole  representa- 
tives of  the  parent  colony  during  midwinter  may  be  the 
sparks  of  life  locked  up  within  the  statoblasts  buried 
in  the  mud. 

Similarly,  the  asexual  spores  of  many  plants,  such 
as  molds,  mosses  and  ferns,  may  be  regarded  as  gem- 
mules  reduced  to  the  lowest  terms,  namely,  to  single 
cells.  As  in  the  preceding  cases  so  in  this  instance  the 
resemblance  of  the  offspring  which  may  arise  from  these 
spores,  to  the  parents  which  produced  them,  is  due  to 
the  essential  material  identity  of  two  generations. 

These  illustrations  of  heredity  in  its  simplest  mani- 
festations give  the  key  to  "organic  resemblance"  higher 
up  in  the  scale.  Sexual  reproduction  is  no  less  plainly 
the  direct  continuation  of  life  though  in  this  instance 
two  sporelike  fragments  out  of  one  generation  con- 
tribute to  form  the  new  individual  of  the  next  genera- 
tion instead  of  one  fragment.  In  all  cases  there  is  a 
material  contmmty  between  succeeding  generations. 
Offspring  become  thus  an  extension  of  a  single  parent, 
or  of  two  parents,  while  heredity  is  simply  "organic 
resemblance  based  on  descent." 

5.  SOMATOPLASM  AND  GERMPLASM 

In  forms  that  reproduce  sexually  there  occurs  a 
differentiation  of  the  body  substance  into  what  Weis- 
mann  terms  somatoplasm  and  germplasm. 


INTRODUCTION  13 

*  Somatoplasm  includes  the  body  tissues,  that  is,  the 
bulk  of  the  individual,  which  is  fated  in  the  course  of 
events  to  complete  a  life-cycle  and  die.  Germplasm, 
on  the  contrary,  is  the  immortal  fragment  freighted  ^ 
with  the  power  to  duplicate  the  whole  organism  and 
which,  barring  accident,  is  destined  to  live  on  and 
give  rise  to  new  individuals. 

Germplasm  thus  carries  potencies  for  developing  both 
germplasm  and  somatoplasm,  while  somatoplasm,  ac- 
cording to  this  conception,  has  only  the  power  to  repair 
itself  but  not  to  reproduce  a  new  individual.  More- 
over, germplasm  is  not  freshly  formed  in  each  genera- 
tion, neither  does  it  arise  anew  when  the  individual  i 
reaches  sexual  maturity,  as  it  appears  to  do,  but  it  is 
a  continuous  substance  present  from  the  beginning. 
Although  this  theory  of  the  contvnuity  of  the  germ- 
plasm  has  been  actually  demonstrated  in  comparatively 
few  instances,  all  the  facts  we  know  concerning  the  be- 
havior of  the  germinal  substance  are  consistent  with  it. 

The  phrase  "life  everlasting"  is  not  confined,  there- 
fore, to  the  vocabulary  of  the  theologian,  and  poten- 
tial immortality  is  more  than  a  mystical  hope  of  be- 
lieving humanity.  They  are  based  upon  demonstrable 
biological  facts, 

In  many  of  the  Protozoa  the  entire  organism  is 
possibly  comparable  to  germplasm,  but  in  all  forms  of 
life  that  are  compounded  of  several  cells  the  germplasm 
is  probably  set  aside  early  in  the  development  of  the 
individual,  and  this  remains  undifferentiated,  or  in  re- 
serve, like  a  savings-bank  account  put  by  for  a  rainy 
day,  while  the  somatoplasm  is  expended  in  the  imme- 


GENETICS 


diate  demands  of  the  tissues  that  make  up  the  indi- 
vidual.    In  one  instance  at  least,  that  of  the  nematode 

worm  As  cans, 
according  to 
Boveri,  this 
splitting  off  or 
isolation  of  the 
germplasm  oc- 
curs as  early  in 
thei  cleavage  of 
the  fertilized 
egg  as  the  six- 
teen-cell  stage, 
when  fifteen  of 
the  cells  go  to 
form  the  soma- 
toplasm  and  the 
sixteenth  is  set 
aside  as  germ- 
plasm. 

Thus  there 
results  a  con- 
tinuous stream 
of  germplasm, 
receiving  con- 
tributions from 
other  germ- 
plasmal  streams 
at  the  time  of 


X 

Germplasm  \  Somatoplasm 

7^ 

FIG.  3. — Scheme  to  illustrate  the  continuity 
of  the  germplasm.  Each  triangle  repre- 
sents an  individual  made  up  of  germ- 
plasm  (dotted)  and  somatoplasm  (un- 
dotted).  The  beginning  of  the  life  cycle 
of  each  individual  is  represented  at  the 
apex  of  the  triangle  where  germplasm  and 
somatoplasm  are  both  present.  As  the 
individual  develops  each  of  these  compo- 
nent parts  increases.  In  sexual  reproduc- 
tion the  germplasms  of  two  individuals 
unite  into  a  common  stream  to  which  the 
somatoplasm  makes  no  contribution.  The 
continuity  of  the  germplasm  is  shown  by 
the  heavy  broken  line  into  which  run  col- 
lateral contributions  from  successive  sex- 
ual reproductions. 


sexual    reproduction,    as    shown   diagrammatically   in 
Figure  3,  in  which  individuals  are  represented  by  tri- 


INTRODUCTION  15 

angles.  From  this  continuous  stream  of  germplasm 
there  split  off  at  successive  intervals  complexes  of  so- 
matoplasm,  or  "individuals,"  which  go  so  far  on  the 
road  of  specialization  into  tissues  that  the  power  to 
be  "born  again"  is  lost,  and  so  after  a  time  they  die, 
while  the  germplasm,  held  in  reserve,  lives  on. 

This  is  what  is  meant  by  saying  that  a  father  and 
son  owe  their  mutual  resemblance  to  the  fact  that  they 
are  chips  off  the  same  block  rather  than  by  saying  that 
the  son  is  a  chip  off  the  paternal  block.  Both  somato- 
plasms  are  developments  at  different  intervals  from 
the  same  continuous  stream  of  germplasm  instead  of 
one  somatoplasm  derived  from  a  preceding  one.  As 
a  matter  of  fact  the  germplasm  from  which  the  son 
arises  is  modified  by  the  addition  of  a  maternal  contri- 
bution, so  that  father  and  son  in  reality  hold  the  same 
relation  to  each  other  that  half-brothers  do. 

So  far  as  his  body  or  his  somatoplasm  is  concerned, 
the  son  is  younger  than  his  father  but  at  the  same  time 
he  is  older  than  his  father  in  his  germplasm,  because  this 
continuous  line  of  germinal  potentiality  has  a  genera- 
tion longer  span  in  him  than  in  his  parents. 

From  the  point  of  view  of  genetics,  then,  the  real 
mission  of  the  somatoplasm,  which  is  so  marvelously 
differentiated  into  all  the  various  forms  that  we  call 
animals  and  plants,  is  simply  to  serve  as  a  temporary 
domicile  for  the  immortal  germplasm.  Thus  the  parent 
becomes  as  it  were  the  "trustee  of  the  germplasm," 
but  not  the  producer  of  the  offspring,  for  the  soma  is 
after  all  only  the  mechanism  through  which  a  fertilized 
egg  produces  in  due  time  another  fertilized  egg. 


16  GENETICS 

In  the  light  of  these  preliminary  explanations  it  is 
plain  that  the  hopeful  point  of  attack  in  the  science  of 
genetics  must  inevitably  be  the  germplasm  which  is  the 
source,  or  point  of  departure,  in  the  formation  of  each 
new  individual,  rather  than  the  somatoplasm,  which 
represents  the  end  stages  of  the  hereditary  processes. 

This  has  not  been  the  method  of  study  in  the  past. 
The  resemblances  of  the  visible  father  and  son  have 
usually  been  traced  instead  of  the  character  of  their 
unseen  germplasms.  By  following  this  old  method,  in- 
vestigators have  often  been  misled  because  the  visible 
or  apparent  is  not  always  the  true  index  of  what  lies 
behind  it.  A  gray  and  a  white  rabbit,  for  example,  may 
produce  some  offspring  that  are  entirely  black  or 
two  white-flowering  sweet  peas  when  crossed  may  some- 
times produce  purple  blossoms.  Consequently  it  is  a 
great  fallacy  to  affirm  that  always  in  heredity  "like 
produces  like,"  since  the  opposite  is  quite  often  the 
case. 

The  new  heredity,  embodied  in  the  science  of  genetics, 
attempts  to  go  deeper  than  the  surface  appearance 
of  the  somatoplasm.  It  aims  to  get  at  the  source  or  ori- 
gin of  organisms,  that  is,  the  germplasm  which  is  the 
only  connecting  thread  between  succeeding  generations 
of  living  forms  from  the  "unbeginning  past."  It  is 
concerned  not  so  much  with  somatoplasm,  which  repre- 
sents what  the  germplasm  has  done  in  the  past,  as 
with  the  germplasm  itself  and  what  it  can  do  in  the 
future. 


CHAPTER  II 

VARIATION 

1.  THE  MOST  INVARIABLE  THING  IN  NATURE 

IN  the  introductory  chapter  it  was  shown  that  "or- 
ganic resemblance  based  on  descent,"  by  which  is 
meant  heredity,  is  due  principally  to  the  fact  that  off- 
spring are  material  continuations  of  their  parents  and 
consequently  may  be  expected  to  be  like  them.  The 
fact  that  this  is  the  case  in  the  great  majority  of  in- 
stances has  given  rise  to  the  popular  formula,  "like 
produces  like,"  as  a  rule  of  heredity. 

But  this  formula  by  no  means  always  fits  the  facts. 
Like  often  produces  something  apparently  unlike. 
For  instance,  two  brown-eyed  parents  may  produce  a 
blue-eyed  child,  although  brown-eyed  children  are  more 
usual  from  such  a  parentage.  It  is  a  common  expe- 
rience, indeed,  for  breeders  of  plants  and  animals  to 
meet  with  continual  difficulties  in  getting  organisms  to 
"breed  true." 

On  the  other  hand,  it  is  exactly  these  variations  which 
so  constantly  interfere  with  breeding  true  that  fur- 
nish the  sole  foothold  for  improvement.  If  all  organ- 
isms did  breed  strictly  true,  one  generation  could  not 
stand  on  the  shoulders  of  the  preceding  generation, 
and  there  would  be  no  evolutionary  advance. 

17 


18  GENETICS 

The  most  invariable  thing  in  nature  is  variation. 
This  fact  is  at  once  the  hope  and  the  despair  of  the 
breeder  who  seeks  to  hold  fast  to  whatever  he  has 
found  that  is  good  and  at  the  same  time  tries  to  find 
something  better.  Variation  is  a  veritable  Pandora's 
box  and  the  chaos  that  would  ensue  if  it  were  not  con- 
fined within  certain  predictable  limits  can  hardly  be 
imagined.  Obviously  the  entire  subject  of  variation  is 
intimately  and  inevitably  bound  up  with  any  considera- 
tion of  genetics,  for  when  the  similarities  and  dissimi- 
larities between  succeeding  generations  are  clear,  then 
heredity  can  be  explained. 

2.  THE  UNIVERSALITY  OF  VARIATION 

Much  of  the  variation  in  nature  is  patent  to  the  most 
casual  observer,  but  it  requires  a  trained  eye  to  see  the 
universal  extent  of  many  minor  differences.  A  flock  of 
sheep  may  all  look  alike  to  a  passing  stranger,  but  not 
to  the  man  who  tends  them.  A  dozen  blue  violet  plants 
from  different  localities  might  easily  be  identified  by  the 
amateur  botanist  as  belonging  to  the  same  species  when, 
to  a  specialist  on  the  genus  Viola,  unmistakable  differ- 
ences would  doubtless  be  clearly  apparent. 

"Identical  twins,"  for  example,  constitute  so  marked 
an  exception  to  the  universal  rule  of  variational  differ- 
ence that  they  challenge  the  attention  at  once,  yet  even 
here  upon  critical  examination  there  appears  some  de- 
gree of  variation. 

The  fact  that  every  attempt  at  an  intimate  acquaint- 
ance with  any  group  of  organisms  whatsoever  invariably 


VARIATION  19 

reveals  previously  unrecognized  variations,  indicates 
that  variability  is  much  more  widespread  in  nature 
than  is  commonly  believed. 

The  key  to  Japanese  art,  as  pointed  out  by  Dr. 
Nitobe,  consists  in  being  natural  and  in  faithfully 
copying  nature.  It  is  for  this  reason  that  the  Jap- 
anese artist  makes  each  object  that  he  produces  unique, 
because  nature  herself,  whom  he  strives  to  follow,  never 
duplicates  anything. 

The  Bertillon  system  of  personal  identification 
is  based  upon  the  constancy  of  minor  variations  found 
in  each  individual.  Its  importance  is  shown  in  Figure 
4».  The  faces  of  the  criminals  there  pictured  would 
be  easily  confused  by  the  ordinary  observer,  but  an 
examination  of  their  thumb  prints  shows  unmistakable 
differences  between  these  three  individuals. 

On  the  other  hand  over-emphasis  upon  the  study  and 
analysis  of  variation  may  tend  to  obscure  the  important 
fact  that  parent  and  offspring  in  the  vast  majority  of 
their  characteristics  are  alike. 

3.  KINDS  OP  VARIATION 

A  brief  enumeration  of  some  of  the  kinds  of  variation 
will  reveal  their  diverse  character. 

a.  With  respect  to  their  nature  variations  may  be 
morphological,  physiological,  or  psychological.  Under 
morphological  variations  are  included  differences  in 
shape,  size,  or  pattern  as  well  as  differences  in  number 
and  relation  of  constituent  parts. 

Differences  in  activity  are  of  a  physiological  nature. 


20  GENETICS 

The  kea  parrot,  after  the  introduction  of  sheep  into 
New  Zealand,  changed  from  herbivorous  to  carnivorous 
habits  and  consequently  became  a  pest.  Many  animals 
in  captivity  are  less  fertile  than  when  free,  while  differ- 
ent individuals  are  well  known  to  vary  widely  with  re- 
spect to  their  susceptibility  to  disease.  Nageli,  for 
example,  reports  the  presence  of  tubercles  in  97  per 
cent  of  the  cases  in  five  hundred  autopsies,  although  a 
majority  of  the  deaths  in  question  was  not  due  to 
tuberculosis  at  all,  a  fact  which  indicates  a  great 
diversity  in  the  resistance  of  different  individuals  to  the 
tubercle  bacillus. 

Psychological  variations  in  man,  such  as  those  which 
determine  the  disposition  or  mental  traits  of  individuals, 
are  apparent  to  every  one. 

b.  With  respect  to  their  duplication  variations  may 
be  single  or  multiple.    A  legless  lamb1  is  an  example  of 
a  single  variation  or  "sport."    Four-leaved  clovers,  on 
the  contrary,   are  multiple  for  the  reason   that   this 
variation,  although  not  common,  nevertheless   occurs 
frequently. 

c.  With  respect  to  their  utility  variations  may  be 
useful,  indifferent,  or  harmful  to  the  organism  possess- 
ing them.     Useful  variations  are  of  the  kind  emphasized 
by  Darwin  as  being  effectively  made  use  of  in  natural 
selection.     Indifferent  variations,  on  the  other  hand, 
are  those  which  apparently  do  not  play  an  important 
part  in  the  welfare  of  their  possessor,  such,  for  ex- 
ample, as  the  color  of  the  eyes  or  of  the  hair.    Finally, 
the  degree  of  degeneration  in  certain  organs  may  be 

1  "A  Peculiar  Legless  Lamb."    Stockard.    Biol.  Bull,  xiii,  p.  288. 


VARIATION  21 

cited  as  an  illustration  of  harmful  variations.  The 
amount  of  closure  of  the  opening  from  the  intestine  into 
the  vermiform  appendix  in  man  is  an  example  of  a 
harmful  variation,  since  the  larger  the  opening,  the 
greater  is  the  liability  to  appendicitis. 

d.  With  respect  to  their  direction  in  evolution  varia- 
tions may  be  either  definite  (orthogenetic)  or  indefinite 
(fortuitous). 

Fortuitous  or  chance  variations  in  all  possible  direc- 
tions furnish  the  repertory  of  opportunity,  according  to 
Darwin,  from  which  natural  selection  picks  out  those 
best  adapted  to  survive  in  the  struggle  for  existence. 

Paleontology  furnishes  numerous  instances  of  the 
former  category,  such  as  the  series  of  variations  from 
a  pentadactyl  ancestor,  all  apparently  tending  in  one 
direction,  which  have  culminated  in  the  one-toed  horse. 
The  fact  that  the  paleontologist  deals  historically  with 
a  completed  phylogenetic  series  in  which  the  side  lines 
lack  prominence,  while  the  successful  line  stands  out 
with  distinctness,  makes  it  easy  for  him  to  view  succes- 
sive variations  as  orthogenetic,  that  is,  as  definitely  di- 
rected in  one  course  either  through  intrinsic  (Nageli)  or 
extrinsic  (Eimer)  causes. 

Just  as  sometimes  the  individuals  of  an  apparently 
continuous  series,  as  shown  in  a  museum  collection  of 
similar  insects,  may  be  of  very  diverse  geographical 
origin,  so  genetics  is  not  primarily  concerned  with  re- 
semblances from  generation  to  generation  but  rather 
with  origins  and  continuity. 

e.  With  respect  to  their  source,  variations  may  be 
somatic   or   germinal.      Somatic,    or  body   variations, 


22  GENETICS 

arise  as  modifications  due  to  environmental  factors. 
They  are  individual  differences  which  may  be  quite 
transitory  in  nature,  while  germinal  variations  may 
arise  without  regard  to  the  environment,  are  deep- 
seated,  and  of  racial  rather  than  of  individual  sig- 
nificance. 

f.  With  respect  to  their  normality  variations  may 
fall  within  expected  extremes  and  thus  be  considered 
normal,  or  they  may  be  outside  of  reasonable  expec- 
tations   and   consequently   be   reckoned   as    abnormal, 
as  in  the  case  of  a  two-headed  calf. 

g.  With  respect  to  the  degree  of  their  continuity 
variations  may  form  a  continuous  series,  grading  into 
each  other  by  intermediate  steps,  or  they  may  be  dis- 
continuous  in  character.   An  example  of   continuous 
variation  is  the  height  of  any  hundred  men  one  might 
chance  to   meet,  which  would  probably   represent   all 
intermediate  grades  from  the  highest  among  the  hun- 
dred to  the  lowest. 

On  the  other  hand  the  number  of  segments  in  the 
abdomen  of  a  shrimp,  for  instance,  which  may  be  either 
eight  or  nine  but  cannot  be  halfway  between,  illustrates 
what  is  meant  by  discontinuous  variation.  The  wide- 
spread occurrence  of  this  later  category  of  variations 
has  been  pointed  out  by  Bateson  in  his  encyclopedic 
volume  "On  Materials  for  the  Study  of  Variation." 

h.  With  respect  to  their  character  variations  may  be 
quantitative  or  qualitative.  A  six-rayed  starfish  rep- 
resents a  quantitative  variation  from  the  normal  num- 
ber of  five  rays,  whereas  a  red  variety  of  a  flower  may 
differ  chemically  from  a  blue  variety,  or  a  bitter  fruit 


VARIATION  23 

may  differ  from  a  sweet  fruit  in  a  qualitative  way  de- 
pendent upon  the  chemical  constitution  of  the  fruit  in 
question. 

i.  With  respect  to  their  relation  to  an  average  stand- 
ard variations  may  have  a  fluctuating  distribution 
around  an  arithmetical  mean,  as  when  some  of  the  off- 
spring have  more  and  some  less  of  the  parental  char- 
acter, or  the  variations  in  the  progeny  may  all  center 
about  a  new  average  quite  distinct  from  the  parental 
standard  and  consequently  come  under  the  head  of 
mutations. 

j.  Finally,  and  most  important  in  the  present  con- 
nection, with  respect  to  heritdbility,  variations  may 
possess  the  power  to  reappear  in  subsequent  genera- 
tions, or  they  may  lack  that  power.  It  is  this  aspect  of 
variability  which  bears  most  directly  upon  genetics. 

Other  possible  categories  might  be  mentioned,  but 
a  sufficient  number  have  been  cited  to  show  the  great 
diversity  of  variations  in  general. 

4.     METHODS  OF  STUDYING  VARIATIONS 

Roughly  stated,  there  are  three  ways  of  studying 
variations:  first,  Darwin's  method  of  observation  and 
the  description  of  more  or  less  isolated  cases;  second, 
Galton's  biometric  method  of  statistical  inquiry ;  and 
third,  Mendel's  experimental  method.  The  second  of 
these  methods  will  be  considered  in  this  chapter. 

5.  BIOMETEY 

The  science  of  biometry,  that  is,  the  application  of 
statistical  methods  to  biological  facts,  has  been  de- 


24  GENETICS 

\ 

veloped  within  recent  years.  Sir  Francis  Galton,  Dar- 
win's distinguished  cousin,  may  be  regarded  as  the 
pioneer  in  this  field  of  research,  while  Karl  Pearson  and 
his  disciples  are  representatives  of  the  modern  school 
of  biometricians. 

Although  mathematical  analysis  of  biological  data 
when  not  sufficiently  ballasted  by  biological  analysis 
of  the  same  facts  may  sometimes  lead  the  investigator 
astray,  yet  often  the  only  way  to  formulate  certain 
truths  or  to  analyze  data  of  some  kinds  is  by  resort  to 
statistical  methods.  Biometricians  are  quite  right  in 
insisting  that  it  is  frequently  necessary  to  go  further 
than  the  fact  of  variation,  which  may  be  apparent 
from  the  inspection  of  an  individual  case,  and  to  deal 
with  cumulative  evidence  as  presented  through  statisti- 
cal analysis. 

In  matters  of  heredity,  however,  facts  as  they  occur 
in  single  cases  and  definite  pedigrees  seem  to  offer  a 
more  hopeful  line  of  approach  than  statistical  generali- 
zations. It  is  better  to  become  acquainted  with  the  real 
parent  than  to  evolve  a  hypothetical  "mid-parent" 
mathematically.  In  this  connection  it  is  well  always  to 
bear  in  mind  the  warning  of  Johannsen,  himself  a  past 
master  in  biometry,  when  he  writes :  "3f it  Mathematik 
nicht  als  Mathematik  treiben  wir  unsere  Studien." 

6.  FLUCTUATING  VARIATION 

With  respect  to  any  measurable  character  there 
are  bound  to  be  deviations  from  an  average  condition. 
According  to  the  mathematical  laws  of  chance  these  de- 


VARIATION 


viations  sometimes  are  plus  and  sometimes  minus,  and 
consequently  they  may  be  termed  fluctuating  variations. 
Pearson  gives  as  a  simple  illustration  of  fluctuating 
variation  the  number  of  ribs  present  in  two  sets  of 
beech-leaves,  as  shown  below.  These  sets  were  taken 
from  two  different  trees,  and  each  contains  twenty-six 
leaves. 


NUMBER  OF  RIBS 

13 

14 

15 

16 

17 

18 

19 

20 

TOTAL 

First   tree      .     . 
Second  tree   . 

3 

4 

1 
9 

4 

8 

7 
2 

9 

4 

1 

26 
26 

Total      .     .     . 

3 

4 

10 

12 

9 

9 

4 

1 

It  will  be  seen  that,  while  certain  leaves  might  well 
belong  to  either  tree,  as,  for  example,  those  with  sixteen 
ribs,  the  entire  group  of  leaves  from  either  tree  is  unlike 
that  of  the  other  tree.  In  the  first  instance  the  number 
of  ribs  fluctuates  around  eighteen  as  the  commonest 
kind ;  in  the  second  case,  around  fifteen.  Such  a  differ- 
ence could  not  easily  be  detected  or  expressed  by  any 
other  method  than  the  statistical  one. 

Again,  in  the  case  of  forty-seven  starfishes  all  of 
which  were  collected  from  one  locality  the  variation 
in  the  number  of  rays  proved  to  be,  according  to 
Goldschmidt,  an  amount  indicated  graphically  in 
Figure  5,  where  the  data  are  arranged  in  the  form  of 
of  a  so-called  frequency  polygon  or  curve. 

From  such  a  polygon  certain  constants  may  be  com- 
puted which  conveniently  express  in  a  single  number, 
for  purposes  of  abstract  comparison,  distinctions  that 


26  GENETICS 

otherwise  could  be  handled  only  in  the  most  indefinite 
way. 


List  of  Constants 

-    Arithmetical  Mean  (A.M.)  =  4.9 

Mode  (Af )  =  5 
~_  Average  Deviation  (-A.D.)  =  .52 

Standard  Deviation  (cr)  =  .846 
.    Coefficient  of  Variability  (C.  V.)  =  1 . 72 

Formulae 


30 


25 


20 


15 


10 


A.D. 


S  =  sum 

x  =  deviation  of the  class  from  A.M. 
f  =  number  in  the  class 
_  »  =  totoZ  number 


Number  of  Rays        2T  3  4  5  6  7 

FIG.  5.— The  fluctuating  variability  of  starfish  rays.    From  data  by 
Goldschmidt. 


Thus  in  this  instance  the  arithmetical  mean,  ex- 
pressed by  the  hypothetical  number  4.915,  a  number 
which  of  course  does  not  actually  occur  in  nature,  is 


VARIATION  27 

simply  the  average  number  of  rays  in  forty-seven  star- 
fishes selected  at  random. 

The  mode  which  represents  the  group  containing 
the  largest  number  of  individuals  of  a  kind,  namely, 
thirty  out  of  forty-seven,  is  five  in  this  particular 
polygon.  If  all  individuals  fell  within  the  mode  there 
would  be  no  variation  and  the  polygon  would  become  a 
vertical  line. 

The  average  deviation,  which  is  an  index  of  the 
amount  of  variation  going  on  among  the  starfishes  in 
question,  is  .52.  In  other  words,  .52  is  the  average 
amount  that  each  individual  starfish  deviates  from  the 
arithmetical  mean  of  4.915.  Although  the  one  seven- 
rayed  starfish  which  happens  to  be  in  the  lot  varies 
from  the  standard  of  4.915  to  the  extent  of  2.085 
(7 — 4.915)  rays,  there  are  thirty  five-rayed  starfishes 
which  vary  only  .085  c(5 — 4.915)  of  a  ray,  and  conse- 
quently the  average  of  the  entire  forty-seven  amounts 
to  .52  of  a  ray.  In  another  collection  of  starfishes 
where  either  more  seven-rayed  or  two-rayed  specimens 
might  be  present,  the  average  deviation  would  probably 
be  greater. 

By  computing  the  average  deviation,  therefore,  and 
using  it  as  the  criterion  of  variation,  a  comparison 
of  the  variability  of  organisms  that  have  been  taken 
from  different  localities  or  subjected  to  different  condi- 
tions can  be  definitely  expressed. 

A  measure  of  variability  more  commonly  in  use  by 
biometricians,  because  of  its  relation  to  probable  error, 
is  the  standard  deviation.  This  is  the  square  root 
of  the  sum  of  all  the  deviations  squared  and  their 


28  GENETICS 

frequencies   divided  by  n,   according   to   the   formula 


.. *  7s  (**•?) 
-\  •  -T-> 


in  which  x  represents  the  deviation  of  each  class  from 
the  arithmetical  mean;  f,  the  number  of  individuals  in 
each  separate  class ;  2,  the  sum  of  the  classes ;  and  n, 
the  total  number  of  individuals.1 

In  the  present  instance  the  standard  deviation  is 
.846,  a  number  that  has  valuable  significance  only 
when  brought  into  comparison  with  standard  deviations 
similarly  derived  from  other  groups  of  starfishes. 

Such  a  variation  polygon  as  the  above  expresses  the 
law  that  the  farther  any  single  group  is  from  the 
mean  of  all  the  groups  making  up  the  polygon,  the 
fewer  will  be  the  individuals  representing  it. 

7.  THE   INTERPRETATION   OF   VARIATION   CURVES 
a.  Relative  Variability 

The  statistical  determination  of  the  relative  vari- 
ability of  two  lots  of  organisms  with  respect  to  a  cer- 
tain character  may  be  illustrated  by  the  case  of  the 
oyster-borer  snail,  Urosalpinx  cinereus,  as  seen  in  the 
accompanying  table  on  page  29. 

The  obvious  conclusion  to  be  drawn  from  this  table 
is  that  the  snails  which  were  unintentionally  carried 
from  the  Atlantic  coast  to  California  in  the  transplan- 
tation of  oysters  show  more  variation  in  their  new 
habitat  than  they  did  in  the  old  one  with  respect  to  the 

1  For  directions  explaining  the  use  of  such  formulae  see  Daven- 
port's "Statistical  Methods." 


VARIATION  29 

ATLANTIC  AND  PACIFIC  SHELLS  COMPARED 


NUMBER 

PROB- 

LOCALITT 

OF 

A.M. 

a 

ABLE 

SHELLS 

ERROR 

rWest    Shore      . 

1,001 

58.928 

2.339 

-K0352 

Penzance  Point 

1,002 

61.718 

2.737 

-K0412 

Nobska  Point  . 

1,002 

61.737 

2.152 

±.0324 

Woods 

Nobska  Point  . 

1,001 

61.944 

2.234 

±.0337 

Hole  1 

Nobska  Point  . 

496 

66.944 

2.366 

±.0507 

Barnacle  Beach 

998 

63.932 

2.604 

±.0393 

Big  Wepecket 
.Mid-Wepecket 

1,006 
500 

57.426 
57.606 

2.052 
2.098 

H-.0308 
±.0447 

Average  for  Mass.     . 

61.066 

2.335 

±.0386 

Call-      J 

Belmont    Beds       .     .     . 

1,008 

59.051 

3.023 

±.0454 

f  ornia  ] 

San  Francisco  Bay    .     . 

520 

60.892 

3.361 

±.0703 

Average  for  Cal.  .     . 

59.664 

3.138 

±.0538 

Difference    .... 

.803 

particular  character  measured,  namely,  the  relative  size 
of  the  mouth  aperture  compared  with  the  height  of  the 
entire  shell.1 

A  further  analysis  of  the  data  in  this  particular  case 
shows  that  this  conclusion  is  probably  biologically  in- 
correct, a  discovery  which  does  not  invalidate  it,  how- 
ever, as  an  illustration  of  a  method  of  determining 
relative  variability. 

b.  Bimodal  Polygons 

Sometimes  two  conspicuous  modes  make  their  ap- 
pearance in  a  frequency  polygon,  as  Jennings  found, 
for  example,  in  measuring  the  body  width  of  a  popula- 
tion of  the  protozoan  Paramecium  (Fig.  6). 

^'Variation  in  Urosalpinx."     Walter.     Amer.  Nat.  1910,  Vol. 
XLIV,  pp.  577-594. 


30  GENETICS 

It  was  subsequently  found  that  the  two  modes  in  this 
polygon  were  due  to  the  fact  that  the  material  in 
question  was  a  mixture  of  two  closely  related  species, 
Paramecium  aurelia  and  Paramecium  caudatum,  the 


Number  of 
Individuals 


FIG.  6. — The  body  width  of  a  population  of  the  protozoan  Para- 
mecium,  showing  a  polygon  with  two  modes.  A,  Paramecium 
aurelia.  B,  Paramecium  caudatum.  After  Jennings. 

individuals  of  which  arranged  themselves  around  their 
own  mean  in  each  instance. 

Although  such  an  explanation  does  not  always  turn 
out  to  be  the  right  one,  the  biometrician  is  led  to  suspect 
when  a  two  or  more  moded  polygon  appears  that  he  is 
dealing  with  a  mixture  of  more  than  one  kind  of  ma- 
terial, each  of  which  fluctuates  around  its  own  average. 

Heterogeneous   material,   it    should   be   noted,   does 


VARIATION 


31 


not  always  give  a  bimodal  curve.  For  example,  if  Pear- 
son's two  lots  of  beech  leaves  mentioned  above  are 
mixed  together,  they  form  a  regular  series  from  the 
inspection  of  which  no  one  could  infer  their  double 
origin.  (See  the  heavy  line  in  Figure  7.) 


12 


Number 
ofriba 


14      15     16       17     18      19      20     21 


FIG.  7.— The  ribs  of  leaves  from  two  beech  trees.  When  put 
together  they  form  a  polygon  which  does  not  reveal  its  double 
origin.  From  data  by  Pearson. 


c.  Skew  Curves 

The  direction  in  which  variations  are  tending  may 
sometimes  be  determined  by  the  statistical  method.  As 
an  illustration  of  this  may  be  cited  the  number  of  ray 
florets  in  1000  white  daisies  (Chrysanthemum  leucanthe- 
mum),  500  of  which  were  collected  at  random  by  the 
writer  from  a  small  patch  in  a  swampy  meadow  in 
northern  Vermont,  while  the  other  500  were  selected 
in  the  same  random  manner  upon  the  same  day  from  a 
dry  hillside  pasture  hardly  more  than  a  stone's  throw 


GENETICS 


distant.  Among  these  two  lots  of  daisies  the  number 
of  ray  florets  varies  from  twelve  to  thirty-eight  and 
their  frequency  polygons,  as  shown  in  Figure  8,  form 
what  are  termed  "skew  curves,"  because  the  mode  in 


12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  88  89 
FIG.  8. — Variation  in  the  ray  florets  of  the  white  daisy  (Chrysan- 
themum leucanthemum) .  A,  from  a  swampy  meadow.  JB, 
from  a  dry  hillside  pasture  near  by.  Both  the  curves  are 
"skew"  because  in  each  case  there  is  an  admixture  of  the 
other  type.  The  distinction  between  the  two  types  is  due  to 
heredity  rather  than  to  environment. 

each  case  lies  considerably  to  one  side  of  the  arith- 
metical mean. 

It  will  be  seen  that  lot  A  from  the  swampy  meadow, 
which  in  spite  of  the  greater  fertility  of  the  soil  and 
the  unquestionably  greater  luxuriance  of  the  plants 
themselves,  produced  heads  with  fewer  florets,  fluctu- 


VARIATION  33 

ates  around  the  number  21,  while  the  dry  pasture  popu- 
lation B,  characterized  by  blossoms  which  were  in 
general  noticeably  smaller,  fluctuates  around  the  num- 
ber 34.  The  habitats  of  the  two  lots  were  so  near  to- 
gether, however,  that  there  was  probably  a  considerable 
intermixture  of  the  two  types,  as  shown  by  the  tendency 
of  each  polygon  to  produce  a  second  mode.  Thus  the  A 
polygon  shows  that  there  is  an  increasing  tendency 
or  variability  in  the  twenty-one  floret  type  toward  the 
thirty-four  floret  type,  due  probably  in  this  particular 
instance  to  invasion  resulting  from  the  proximity  of 
the  B  colony. 

8.  GRADUATED  AND  INTEGRAL.  VARIATIONS 

It  is  comparatively  simple  to  treat  statistically 
integral  variations,  illustrations  of  which  have  been 
given  in  the  case  of  beech-leaf  ribs,  starfish  rays,  and 
daisy  florets,  all  of  which  are  characters  that  can  be 
readily  counted.  In  the  same  way  any  measurable 
character,  such  as  the  size  of  snail  shells,  may  fall  into 
easily  limited  groups,  as,  for  example,  10  to  11  mm., 
11  to  12  mm.,  12  to  13  mm.,  etc.  It  is  somewhat 
more  difficult  to  classify  variations  when  color  or 
pattern  is  the  character  in  question,  since  it  then  be- 
comes necessary  to  define  certain  arbitrary  limits  for 
each  class  of  the  series  within  which  to  group  the  indi- 
vidual variants. 

Tower,  in  his  famous  researches  on  potato-beetles, 
encountered  variations  in  the  pigmentation  of  the  pro- 
notum  all  the  way  from  entire  absence  of  color  to  com- 


34  GENETICS 

plete  pigmentation  but  by  cutting  up  this  continuous 
series  of  variations  into  arbitrary  groups  of  equal 
extent,  it  was  quite  possible  to  arrange  the  data  so 
that  they  could  be  statistically  treated  just  as  con- 
veniently as  the  integral  variations  mentioned  above. 
Groups  or  classes  of  this  kind  are  termed  graduated 
variations. 

9.  THE  CAUSES  OF  VARIATION 

With  respect  to  the  causes  of  variation  authoritative 
biologists  have  taken  different  points  of  view. 

a.  Darwin  considered  variations  as  axiomatic.     An 
axiom  is  self-evident,  requiring  no  explanation.     The 
absence  of  variations  in  organisms  rather  than  the  oc- 
currence of  variations  is,  from  this  point  of  view,  the 
phenomenon  requiring  an  explanation.     Although  Dar- 
win himself  spent  some  time  in  pointing  out  the  univer- 
sal occurrence  of  variability,  he  accepted  it  as  a  pri- 
mary fact  and  proceeded  from  it  as  a  starting  point 
without  attempting  to  seek  its  causes. 

b.  Lamarck    and   his    followers   have   regarded   the 
causes  of  variation  either  as  extrinsic,  that  is,  refer- 
able to  external  factors  making  up  the  environment  of 
the  organism,  or  as  intrinsic  or  physiological,  that  is, 
based  upon  the  efforts  which  an  organism  puts  forth 
to  fit  into  its  particular  environment  successfully.    The 
causes  of  variation  are  to  be  sought  according  to  the 
Lamarckian    school,    in   the   "environment"    and    "re- 
sponse" sides  of  the  triangle  of  life  rather  than  in  the 
"heritage"  side  (Fig.  1). 

For  example,  Woltereck,  by  controlling  the  single 


VARIATION  35 

extrinsic  factor  of  food  supply,  was  able  to  modify 
the  height  of  the  "head"  of  the  microscopic  fresh- 
water crustacean,  Hyalodaphnia,  in  the  remarkable 
manner  indicated  in  Figure  9.  When  poor  food  was 
supplied,  the  percentage  of  the  head  height  to  that 
of  the  body  averaged  hardly  forty,  while  with  rich  food 
it  was  increased  to  over  ninety. 

Similarly  Klebs  succeeded  in  changing  at  will  the 


35     40     45     50     55     60     65     70     75     80     85     90     95    100  # 
Ratio  of  height  of  head  to  length  of  shell ' *• 


Fio.  9. — Schematic  curve  of  the  head  height  of  Hyalodaphnia 
under  various  conditions  of  nourishment.  Adapted  from 
Woltereck. 


number  of  stamens  in  the  common  "live-for-ever," 
Sedum  spectabile,  by  manipulating  the  environment  in 
which  the  plants  were  kept.  Some  of  his  results  are 
shown  in  Figure  10.  Curve  A  combines  the  data  for 
4260  flowers  which  were  raised  in  well-fertilized  dry 
soil  under  bright  light ;  curve  B  represents  4000  flowers 
grown  in  a  moist  greenhouse  under  red  light ;  and 
curve  C  includes  4390  flowers  from  well-fertilized  soil 
in  moist  hotbed  conditions  under  a  weak  light. 

c.  Weismawi,    on   the    contrary,   believes    that    the 


36  GENETICS 

causes  of  variation,  at  least  of  heritable  variations, 
are  intrinsic  or  inborn  in  the  germplasm.  His  con- 
ception of  sexual  reproduction  is  that  it  is  a  device  for 
doubling  the  possible  variations  in  the  offspring  by 
the  mingling  of  two  strains  of  germplasm  (amphimixis). 
By  far  the  greater  number  of  observations  recorded 
go  to  substantiate  this  theory. 

A 

Number  of 
Flowers 
80 1 

70 


10  98765    10  9876543  10  987654 

Number  of  Stamens 

FIG.  10. — Variations  in  the  number  of  stamens  in  the  flowers  of 
the  "live-for-ever"  (Sedum  spectabile)  under  various  con- 
trolled conditions.  For  detailed  description,  see  text.  After 
Klebs. 

Tower  found  among  his  potato-beetles,  for  example, 
that  two  strains  reared  in  the  same  environment  showed 
striking  differences  in  variation,  a  fact  necessarily  due 
to  intrinsic  rather  than  to  extrinsic  factors.  Similar 
cases  may  be  recalled  by  any  one. 

Nevertheless,  heritable  variation  occurs  in  the  ab- 
sence of  amphimixis  so  that,  at  best,  sexual  reproduc- 
tion furnishes  only  one  of  the  possible  avenues  for  the 
introduction  of  hereditary  variations. 


VARIATION  37 

d.  Lastly,  Bateson,  whose  work  "On  Materials  for 
the  Study  of  Variation"  already  cited  is  a  classic, 
takes  the  agnostic  attitude  that  it  is  rather  futile  to 
guess  at  the  causes  of  variation  before  the  facts  are 
well  in  hand.  He  consequently  discourages  such  at- 
tempts by  saying:  "Inquiry  into  the  causes  of  varia- 
tion is,  in  my  judgment,  premature." 

In  conclusion,  the  words  of  Darwin  written  over  half 
a  century  ago :  "Our  ignorance  of  the  laws  of  variation 
is  profound,"  may  still  be  appropriately  quoted,  not- 
withstanding the  fact  that  in  biometry  we  have  at  least 
an  excellent  analytical  method  by  means  of  which  con- 
siderable insight  into  variation  is  being  gained. 


CHAPTER  III 

HERITABLE  DIFFERENCES 

1.  THE   MUTATION   IDEA 

VARIETY  is  not  only  the  "spice  of  life"  but  it  is  also 
the  central  necessity  in  the  origin  of  new  kinds  of  ani- 
mals and  plants.  If  there  was  no  variation  from  gen- 
eration to  generation  then  nothing  new  would  appear 
which  nature  could  in  any  way  seize  upon  in  order  to 
escape  from  conservative  monotony  and  as  a  result 
there  would  be  no  possible  evolution  in  any  direction. 
This  deplorable  state  of  affairs  we  know  is  contrary  to 
fact. 

There  are  at  least  three  ways,  according  to  Baur, 
by  which  an  organism  can  become  different  from  its 
relatives,  viz. — 1,  modification;  £,  combination;  3, 
mutation.  Which  of  these  three  ways  has  been  followed 
in  any  specific  instance  can  only  be  determined  with 
certainty  by  the  test  of  subsequent  breeding,  for  there 
is  nothing  in  the  appearance  of  an  animal  or  plant  to 
indicate  by  which  of  these  three  paths  it  has  gained 
any  distinctive  variation. 

By  modifications  we  understand  those  widespread 
differences  which  are  the  result  of  nurture  rather  than 
nature.  They  are  simply  environmental  effects  upon 
the  somatoplasm  and  consequently  are,  in  all  probabil- 

38 


HERITABLE  DIFFERENCES  39 

ity,  transitory  so  far  as  their  inheritance  is  concerned. 
They  are  the  result  of  soil  rather  than  seed. 

"Combinations"  and  "mutations"  are  more  deep- 
seated.  They  are  conditioned  by  the  germinal  nature 
of  the  organism  and  may,  therefore,  be  passed  on  as 
hereditary. 

Combinations  are  the  result  of  a  new  deal  after  a 
reshuffling  of  the  cards.  Nothing  essentially  new,  which 
was  not  already  present  in  one  or  the  other  of  the 
parental  lines,  is  introduced  but  a  different  arrange- 
ment or  bringing  together  of  old  qualities  is  effected. 
This  process  of  variation  through  hybridization  is  the 
concern  of  Mendelism  and  will  be  considered  further  on. 

Mutations,  like  Minerva  springing  full-fledged  from) 
the  head  of  Jove,  are  something  qualitatively  new  which' 
appear  abruptly  without  transitional  steps  and  that! 
breed  true  from  the  very  first. 

A  distinctive  qualitative  character  marks  mutations, 
like  the  discontinuous  differences  between  such  chemical 
compounds  as  carbon  monoxide  (CO)  and  carbon 
dioxide  (CO2),  as  Bateson  has  pointed  out,  but  the 
leap  from  one  to  the  other  may  be  so  small  that  it  is 
difficult  to  ascertain  by  inspection  whether  the  differ- 
ence is  due  to  mutation  or  to  modification.  The  test 
comes  in  breeding,  since  the  progeny  of  a  modification, 
or  "fluctuation"  as  deVries  terms  it,  will  revert  to  the 
old  average  of  the  parental  generation,  while  the 
progeny  of  a  mutation  will  vary  around  a  new  average 
set  by  the  mutation  itself. 

The  series  of  positions  taken  by  the  lower  end  of  a 
swinging  pendulum  illustrate  what  is  meant  by  these 


40  GENETICS 

non-heritable  fluctuating  modifications.  They  all  hold 
predictable  relations  to  the  average  position  shown 
when  the  pendulum  comes  to  rest,  because  whenever 
the  pendulum  is  put  in  motion  the  various  positions 
all  recur  as  before.  A  mutation,  on  the  contrary,  is 
represented  by  a  change  in  the  point  of  attachment  at 
the  upper  end  of  the  pendulum.  It  occurs  only  when 
the  entire  pendulum  is  unhooked  and  hung  up  in  a 
different  place.  This  new  point  of  attachment  must  be 
chosen  arbitrarily  and  has  no  such  definite  relation  to 
the  original  attachment  as  characterizes  the  variation 
in  position  of  the  swinging  end  of  the  pendulum. 

When  the  attempt  is  made  to  arrange  a  series  of  suc- 
cessive mutations  in  a  curve  they  do  not  show  a  graded 
relationship  to  each  other  as  fluctuations  do.  The 
latter  mass  around  the  average  standard  according  to 
the  laws  of  chance  in  much  the  same  way  that  a  hundred 
shots  by  a  good  marksman  may  center  around  a  bull's- 
eye.  Mutations  never  group  in  this  way.  They  find  no 
correspondence  even  with  wild  shots  at  the  bull's-eye. 
They  are  shots  directed  at  a  different  target  altogether. 
To  use  the  musician's  phraseology,  a  variation  elabo- 
rated upon  an  old  theme  would  correspond  to  a  modi- 
fication but  a  mutation  would  be  an  entirely  new 
theme. 

Darwin  was  fully  aware  of  the  existence  of  mutations 
or  "sports"  as  he  called  them,  and  incidentally  gave 
time  to  their  consideration,  but  the  great  task  which 
he  set  out  to  accomplish  in  such  a  masterly  manner 
was  to  overthrow  the  widespread  and  deep-seated  be- 
lief of  his  day  in  a  sudden  special  creation  of  distinct 


HERITABLE  DIFFERENCES  41 

species.  To  this  end  he  marshaled  evidence  in  support  of 
the  gradual  transition  of  one  species  into  another, 
emphasizing  fluctuating  modifications  rather  than  muta- 
tions which  seemed  to  him  to  play  a  minor  role  in  the 
origin  of  species. 

It  remained  for  the  Dutch  botanist  Hugo  deVries  to 
be  the  first  to  analyze  the  character  of  mutations  and 
to  focus  attention  upon  them.  There  is  something 
distinctly  suggestive  of  Darwin's  method  in  the  fact 
that  deVries  worked  in  silence  for  twenty  years  before 
he  gave  the  world  the  "Mutationstheorie"  with  which 
his  name  will  be  forever  connected. 


2.  A  SUMMARY  OF  THE  MUTATION  THEORY 

The  main  features  of  the  mutation  theory  of  deVries 
may  be  indicated  as  follows: — 

a.  New  species  arise  abruptly  regardless  of  environ- 
ment without  transitional  forms,  and  at  present  they 
are  not  known  to  arise  in  any  other  way. 

6.  New  forms  arise  as  unusual  deviations  from  the 
parent  form,  which  itself  remains  unchanged  although 
it  may  repeatedly  give  rise  to  similar  deviations. 

c.  New  mutations  are,  from  the  first,  constant,  that 
is,    they    produce    their   like.      They    do    not    become 
gradually   evolved   as   the   result   of  natural   selection 
although  natural  selection  may  act  upon  them  after 
they  appear. 

d.  Among  mutations  there  may  occur  forms  char- 
acterized by  the  addition  of  something  new, — progres- 
sive   elementary    species, — as    well    as    forms    lacking 


42  GENETICS 

something  present   in   the   parental   type, — regressive 
varieties. 

e.  The  same  mutation  may  arise  simultaneously  in 
many  individuals  instead  of  as  a  single  "sport." 

f.  Mutations   do  not  vary  around   an   arithmetical 
mean  with  respect  to  the  parent  form,  as  is  the  case 
with  fluctuating  variations,  but  each  fluctuates  around 
a  new  average  of  its  own,  thus  forming  a  discontinuous 
series  with  the  parent  form. 

g.  Mutations  may  occur  in  all  directions,  that  is, 
they  are  not  necessarily  definite  or  orthogenetic. 

h.  Mutations  probably  appear  periodically. 

i.  Every  mutation  means  two  possible  species  where 
one  existed  before. 

j.  Useless  or  insignificant  fluctuating  variations  are 
not  necessarily  the  material  from  which  natural  selec- 
tion must  sift  out  new  species. 

k.  Natural  selection  is  not  a  causative  agent  in 
evolutionary  advance  but  is  simply  a  sieve  which  picks 
out  successful  survivors  from  mutations. 

3.  LAMARCK'S  EVENING  PRIMROSE 

Perhaps  the  most  widely  known  plant  mutations  are 
the  progeny  of  Lamarck's  evening  primrose,  (Enothera 
lamarckiana,  because  it  was  these  plants  that  led 
deVries  to  formulate  his  mutation  theory. 

It  is  believed  by  botanists  in  general  that  this  plant 
is  a  native  of  the  southern  United  States,  although, 
so  far  as  is  known,  it  is  now  extinct  as  a  wild  species 
in  America,  and  native  specimens  are  included  in  but 
few  American  herbaria. 

It  was  exported  to  London  as  a  garden  plant  about 


HERITABLE  DIFFERENCES  43 

1860,  and  thence  it  spread  to  the  continent,  where, 
escaping  from  gardens,  it  became  wild  in  at  least 
one  locality  near  Hilversum,  a  few  miles  from  Amster- 
dam. Here,  in  an  abandoned  potato  field,  it  fell  under 
the  seeing  eye  of  Hugo  deVries  in  1885,  and  now  both 
botanist  and  primrose  are  famous. 

DeVries  found  among  these  escaped  plants  not  only 
0.  lamarckiana,  but  also  two  other  kinds  of  mutants, 
0.  brevistylis,  characterized  by  short-styled  flowers, 
and  0.  Icevifolia,  which  has  smooth  leaves.  These  two 
were  entirely  new  species  hitherto  unknown  at  the  great 
botanical  clearing-houses  of  Paris,  Leyden,  and  the 
Kew  Gardens. 

Since  the  seeds  of  the  (Enofhera  are  produced  by 
self-fertilized  flowers,  deVries  felt  safe  in  regarding 
these  plants  as  mutants  rather  than  hybrids,  and  he 
continued  to  study  them  with  especial  care.  Trans- 
planting the  mutants  along  with  representatives  of  0. 
lamarckicma  to  his  private  gardens  in  Amsterdam, 
where  it  was  possible  to  maintain  them  in  normal 
healthy  condition,  deVries  was  able  to  follow  their  indi- 
vidual histories  with  certainty. 

The  wild  mutants  Icevifolia  and  brevistylis  did  not 
reappear  under  cultivation  but  he  found  that,  out  of 
54,343  plants  of  the  species  0.  lamarckiantt  grown  as 
descendants  from  nine  original  plants  during  eight 
years,  there  appeared  837  mutants  comprising  seven 
different  elementary  species,  all  of  which,  with  the  ex- 
ception of  0.  scintttlans,  bred  true.  See  table  on  the 
next  page. 

Some  explanatory  comment  on  this  table  may  be 
of  value. 


44 


GENETICS 

MUTANTS  or  CENOTHERA  LAMARCKIANA 


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GENERATION 

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1886-7 

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II 

1888-9 

15,000 

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III 

1890-1 

1 

10,000 

3 

3 

IV 

1895 

1 

15 

176 

8 

14,000 

60 

73 

1 

V 

1896 

25 

135 

20 

8,000 

49 

142 

6 

VI 

1897 

11 

29 

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1,800 

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1898 

9 

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54,343 

The  seeds  in  each  generation  were  self-fertilized 
lamarMana. 

The  mutant  gig  as  occurred  once,  in  1895.  From 
the  seeds  of  this  one  plant  were  produced  450  true 
gigas  offspring  in  the  first  year,  and  the  strain  con- 
tinues to  breed  true. 

Albida  was  first  noted  in  1895,  but  deVries  remem- 
bered having  seen  it  before  and  dismissing  it  as 
pathological.  Because  of  its  poverty  in  chlorophyll  it 
is  a  mutant  which  probably  would  not  maintain  itself 
successfully  in  nature,  but  it  breeds  constant  under 
cultivation. 

Oblonga  always  bred  true  with  the  exception  of 
throwing  an  albida  in  1895  and  a  single  example  of 
rubrinervis  in  1899. 

Of  rubrinervis  over  2000  invariably  bred  true,  while 
nanella  bred  true  in  over  20,000  offspring,  with  but 
three  exceptions  when  oblonga  characters  appeared. 


HERITABLE  DIFFERENCES  45 

Lata,  since  it  produces  only  female  flowers  and  so 
cannot  be  self-fertilized,  had  constantly  to  be  crossed 
back  with  the  parent  lamarckiana,  when  it  produced 
from  15  to  20  per  cent  lata  and  80  to  85  per  cent 
lamarckiana. 

Finally,  scmtillans  which  appeared  at  three  separate 
times  proved  constant  only  in  its  inconstancy  because 
it  invariably  produces  a  heterogeneous  progeny.  The 
1895  plant  gave  53  per  cent  lamarckiana,  35  per  cent 
scintillans,  10  per  cent  oblonga,  and  1  per  cent  lata. 
One  of  the  1896  plants  gave  51  per  cent  lamarckiana, 
39  per  cent  scmtillans,  8  per  cent  oblonga,  1  per  cent 
lata,  and  1  per  cent  nanetta,  while  another  1896  plant 
gave  only  8  per  cent  lamarckiana,  but  69  per  cent 
scmtillans,  21  per  cent  oblonga,  and  2  per  cent  of 
nanella  and  lata  together. 

These  different  kinds  of  evening  primroses  are  dis- 
tinguished from  each  other  by  features  which  are  un- 
mistakable even  to  the  uninitiated.  The  old-time  sys- 
tematist  would  undoubtedly  have  regarded  them  as  dis- 
tinct species. 

DeVries  distinguishes  four  categories  among  the 
CEnothera  mutants,  the  first  three  of  which  are 
quite  likely  to  maintain  themselves  in  nature.  They 
are: — 

1.  Progressive  species,  (gig as,  rubrinervis) ,  due  to 

the  addition  of  certain  characteristics ; 

2.  Retrogressive  varieties,    (nanella,  Iceviflora,   bre- 

vistylis),    characterized  by   the  loss   of   some- 
thing that  was  present  in  the  parent  form; 


46  GENETICS 

3.  Inconstant  species,  (scintillans  and  lamarckiana 

itself),  that  do  not  always  breed  true  but  pro- 
duce mutants,  and 

4.  Degressive  species,  (lata,  albida),  which  are  de- 

fective in  some  way  and  are  incapable  of  main- 
taining themselves  in  nature. 

DeVries'  experiments  and  observations  have  been 
repeated  on  a  large  scale  and  extended,  notably  by 
MacDougal  in  the  New  York  Botanical  Gardens,  by 
Shull  at  the  Carnegie  Institution  for  Experimental 
Evolution,  Cold  Spring  Harbor,  Long  Island,  and  by 
Gates  in  England,  and  his  conclusions  have  been  con- 
firmed in  all  essential  points.  The  mutability  of  0. 
lamarckiana  is  as  unmistakable  and  as  diverse  in 
America  and  England  as  it  is  in  Holland. 

The  critics  of  deVries,  however,  regard  (Enothera 
lamarckiana  as  a  hybrid  to  begin  with,  from  which 
different  strains  have  simply  been  bred  out.  Both 
Bateson  and  Lotsy  have  called  attention  to  the  pres- 
ence of  deformed  or  defective  germ  cells  in  (Enothera 
lamarckiana  as  evidence  of  its  hybridity,  and  Davis, 
by  crossing  0.  franciscana  and  O.  biennis,  has  pro- 
duced a  hybrid  (Enothera,  which  he  has  christened 
(Enothera  neo-lamarckiana  because  it  not  only  resem- 
bles 0.  lamarckiana  but  behaves  like  it  in  producing 
mutations.  He  consequently  proposes  "dissolution  of 
hybrids"  as  a  substitution  for  mutation  in  explaining 
the  phenomena  that  deVries  has  described. 

It  is  somewhat  questionable  whether  this  classical 
plant,  which  has  added  at  least  a  five-foot  shelf  to 


HERITABLE  DIFFERENCES  47 

the  biological  literature  of  the  last  thirty-five  years, 
is  after  all  the  most  fortunate  organism  for  demon- 
strating mutation  since  its  "mutations"  may  represent 
simply  combinations  becoming  isolated  from  something 
already  present  as  the  result  of  past  hybridization. 
In  either  case  the  new  form  would  breed  true  and  behave 
like  a  true  mutation. 


4.  PLANT  MUTATIONS  FOUND  IN  NATURE 

The  oldest  known  authenticated  case  of  a  plant 
mutation  is  the  oft  cited  instance  of  the  "fringed 
celandine,"  Chelidonium  laciniatwm,  which  made  its 
appearance  in  the  garden  of  the  Heidelberg  apothe- 
cary Sprenger  in  1590  among  plants  of  the  "greater 
celandine,"  Chelidonium  majus.  The  fringed  celan- 
dine bred  true  at  once  and  is  now  a  widespread  and 
well-known  species. 

The  purple  beech  has  appeared  historically  as  a 
mutant  among  ordinary  beeches  upon  at  least  three 
occasions  in  widely  separated  localities,  and  it  has 
always  given  rise  to  a  constant  progeny. 

The  "Shirley  poppy,"  notable  for  its  remarkable 
range  of  color,  which  was  discovered  in  1882  by  Rev. 
W.  Wilks,  originated  from  a  single  plant  of  the  small 
red  poppy,  Papaver  rlioeas,  which  is  commonly  found 
in  English  cornfields. 

The  first  double  petunia  was  found  in  1855  in  a 
private  garden  in  Lyons.  (Ziegler.)  Other  instances 
are  known  of  double  flowers  among  roses,  azaleas, 
stocks,  carnations,  primroses,  etc.,  arising  from  single 


48  GENETICS 

flowering  plants,  the  seeds  of  which  in  turn  produce 
double  flowers. 

The  giant  primrose  is  a  mutation  from  a  normal 
strain  of  known  pedigree.  (Keeble.) 

"Mutations  in  certain  pericarp  color  patterns  of 
maize  are  so  common  that  a  wide  range  of  variability 
results.  Selection  is  able  from  such  material  to  iso- 
late types  relatively  stable  but  very  diverse  in  appear- 
ance." (Emerson  and  Hayes.) 

That  plant  mutations  may  occur  in  nature  and  per- 
sist successfully  without  isolation  or  external  selection 
is  shown,  for  instance,  by  Schaffner  1  who  reports  an 
unusual  white  verbena  growing  wild  in  Ohio  over  about 
a  square  mile  of  territory  along  with  the  typical  pur- 
plish blue  Verbena  stricta  without  transitional  forms. 

Hayes  discovered  a  tobacco  mutant  in  which  the 
average  number  of  leaves  produced  was  70  instead  of 
20,  and  Cockerell  found  a  single  red  mutant  plant  of 
the  sunflower,  Helianthus  lenticuLaris  coronatus,  which 
has  bred  true.  The  list  of  similar  plant  mutations 
could  be  almost  indefinitely  extended. 

5.  SOME  MUTATIONS  AMONG  ANIMALS 

In  1791  a  Massachusetts  farmer,  by  name  Seth 
Wright,  found  in  his  flock  of  sheep  a  male  lamb  with 
long,  sagging  back  and  short,  bent  legs  resembling 
somewhat  a  German  dachshund.  With  unusual  fore- 
sight he  carefully  brought  up  this  strange  lamb  be- 
cause it  was  an  animal  that  could  not  jump  fences. 

'Ohio  Naturalist.     Dec.,  1906. 


HERITABLE  DIFFERENCES  49 

It  occurred  to  this  hard-headed  Yankee  that  it  would 
be  much  easier  to  get  together  a  flock  of  short,  bow- 
legged  sheep,  unable  to  negotiate  anything  but  a  low 
hurdle,  than  to  labor  hard  at  building  high  fences. 
So  it  came  about  that  this  mutating  lamb,  in  the  hands 
of  a  man  who  appreciated  labor-saving  devices,  became 
the  ancestor  of  the  Ancon  breed  of  sheep.  Later  on 
this  breed  gave  place  in  public  favor  to  another  mu- 
tant, the  Merino,  which  produces  a  superior  grade  of 
wool. 

Some  mutations,  however,  that  may  be  selected  and 
maintained  by  man  are  unlikely  to  succeed  in  nature 
when  left  to  themselves.  Albino  animals,  for  example, 
are  so  handicapped  by  defective  eye-sight  that  they 
have  a  hard  struggle  in  the  wild  condition.  Albino 
rats  set  free  by  Dr.  Hatai  a  few  years  ago  upon  Goose 
Island,  a  small  uninhabited  bit  of  land  in  Long  Island 
Sound,  all  succumbed  to  the  native  rats  in  a  short 
time. 

Hornless  cattle  suffer  fewer  injuries  from  one  an- 
other than  horned  cattle.  It  has  consequently  become 
quite  a  general  practice  among  farmers  to  "dehorn" 
their  stock  surgically.  It  is  an  obvious  advantage  to 
have  cattle  born  hornless,  and  many  breeds  having 
this  character  are  now  established.  In  1889  a  mutant 
among  horned  stock  appeared  at  Atchison,  Kansas, 
in  the  form  of  a  hornless  Hereford.  From  this  mutant 
has  descended  the  well-established  race  of  polled  Here- 
ford cattle,  constituting  a  bovine  aristocracy  with 
registry  books  and  blue  blood  all  their  own. 

Taillessness  in  cats,  dogs  and  poultry,  as  well  as 


50  GENETICS 

hairlessness  in  cattle,  dogs,  mice  and  horses,  are  fur- 
ther instances  of  mutations. 

Davenport,1  writing  of  his  experiments  with  poultry, 
says:  "During  the  past  four  years  I  have  handled 
and  described  over  10,000  poultry  of  known  ancestry. 
Of  striking  new  characters  I  have  observed  many, 
some  incompatible  with  normal  existence;  others  in 
no  way  unfitting  the  individual  for  continued  life. 
In  the  egg  unhatched  I  have  obtained  Siamese  twins, 
pug  jaws,  and  chicks  with  thigh  bones  absent.  There 
have  been  reared  chicks  with  toes  grown  together  by 
a  web,  without  toenails  or  with  two  toenails  to  a  toe; 
with  five,  six,  seven,  or  three  toes;  with  one  wing  or 
both  lacking;  with  two  pairs  of  spurs;  without  oil- 
gland  or  tail ;  with  neck  devoid  of  feathers ;  with  cere- 
bral hernia  and  a  great  crest;  with  feather  shaft  re- 
curved, with  barbs  twisted  and  dichotomously  branched 
or  lacking  altogether.  Of  comb  alone  I  have  a  score 
of  forms.  All  of  these  characters  have  been  offered 
to  me  without  the  least  effort  or  conscious  selection  on 
my  part,  and  each  appeared  in  the  first  generation  as 
well-developed  peculiarities,  and  in  so  far  as  their 
inheritance  was  witnessed,  each  refused  to  blend  when 
mated  with  a  dissimilar  form." 

Bateson  (1894),  in  his  "Materials  for  the  Study  of 
Variation,"  gives  a  detailed  list  of  886  cases  of  "dis- 
continuous variations"  among  animals,  many  of  which 
doubtless  belong  to  the  category  of  mutations,  al- 

1  Davenport,  C.  B.,  1909.  "Inheritance  of  Characteristics  in 
Domestic  Fowl."  Carnegie  Institution  of  Washington,  Publica- 
tion No.  131. 


HERITABLE  DIFFERENCES  51 

though  several  may  be  "combinations"  or  must  be 
placed  even  in  the  non-inheritable  class  of  "freaks." 

The  chief  reason  why  definite  examples  of  mutation 
are  so  infrequently  noted  and  recorded  is  because 
the  attention  of  the  investigator  has  generally  been 
directed,  not  to  them,  but  to  gradual  fluctuating  varia- 
tions which,  according  to  Darwin's  conception,  furnish 
the  material  for  the  operation  of  natural  selection. 
Mutations  are  doubtless  much  more  common  than  has 
been  generally  supposed,  and  it  is  likely  that  they  will 
receive  more  attention  in  the  future  than  they  have  in 
the  past. 

No  stock  when  bred  on  a  large  scale  breeds  abso- 
lutely true  for  all  specific  characters.  Gerould  re- 
ports that  in  his  butterflies  (Colias),  he  found  blue- 
green  instead  of  yellow-green  eyes,  uncoiled  instead  of 
coiled  tongue,  the  absence  of  orthodox  wing  spots, 
one  proleg  less  in  the  caterpillar,  etc.  Drosophila 
is  a  famous  example  of  many  deviations  from  type 
which  have  been  revealed  upon  persistent  and  careful 
scrutiny. 

6.  KINDS  OF  MUTATION 

Multiple  or  aggregate  mutations  are  those  germinal 
upsets  that  affect  many  parts  of  an  organism  instead 
of  a  single  part.  This  type  is  of  frequent  occurrence 
and  is  in  contrast  to  a  single  gene  mutation  which  in- 
volves only  an  hereditary  unit  that  determines  a  single 
somatic  feature.  For  example,  Babcock  describes  a 
new  walnut,  Juglans  quercina,  which  appeared  inde- 


52  GENETICS 

pendently  in  four  different  widely  separated  localities 
in  California.  This,  like  deVries'  evening  primrose, 
was  an  aggregate  mutation,  for  differences  appeared 
in  size,  shape,  color  and  texture  of  leaves ;  size,  form 
and  color  of  flower-parts;  color  of  bark,  habit  of 
growth,  etc.  That  this  was  a  true  mutant  and  not  a 
hybrid  between  the  oak  and  the  walnut  was  indicated 
by  negative  results  in  cross-pollinating  experiments. 
Similar  aggregate  mutations  have  been  reported  for 
cotton,  tomato,  tobacco  and  other  organisms. 

Another  phenomenon  that  probably  indicates  com- 
mon ancestral  germplasm  among  species,  at  present 
apparently  independent  of  each  other,  is  the  occurrence 
of  parallel  mutations.  The  North  African  ostrich 
(Struthio  camelus)  and  the  South  Australian  ostrich 
(S.  australis),  although  separated  from  each  other  for 
long  geological  time,  show,  according  to  Duerden, 
similar  mutations  in  size,  length  of  neck  and  legs,  skin- 
color  and  bald-head  as  well  as  in  size  and  shape  of  the 
egg  and  the  character  of  its  surface,  whether  pitted  or 
ivory-smooth. 

A  long  list  of  parallel  mutations  in  Drosophila 
melanogaster  and  D.  virilis  has  been  described  by  Metz, 
and  similarly,  Sturtevant  reiports  mutations  in  D. 
funebris  that  are  likewise  parallel  to  those  of  D. 
melanogaster,  in  which  the  occurrence  of  mutations 
has  probably  been  more  carefully  studied  than  in  any 
other  animal. 

Sumner  with  the  deer-mouse,  Peromyscus,  has  found 
albinism,  spotting  and  red-eyed  yellow,  all  mutations 
known  to  occur  in  other  mice. 


HERITABLE  DIFFERENCES  53 

Reverse  mutations  have  also  been  repeatedly  ob- 
served. This  is  something  resembling  the  unscrambling 
of  an  egg.  Morgan  and  Bridges  obtained,  for  exam- 
ple, normal  red-eyed  flies  from  white-eyed  mutants  and 
May,  also  with  ubiquitous  Drosophila,  got  back  nor- 
mal-eyed individuals  from  bar-eyed  mutants. 

The  frequent  occurrence  of  recurrent  mutations, 
that  is,  the  reappearance  of  the  same  mutations,  sug- 
gests that  the  cause  underlying  these  irregular  heredi- 
tary changes  is  something  continuous  and  definite  even 
if  we  are  at  present  unable  always  to  put  our  finger 
upon  it.  The  evening  primroses  have  repeatedly  shown 
the  same  mutations  in  widely  different  localities  and 
under  the  eyes  of  different  investigators.  Morgan  says 
of  his  famous  banana  flies,  "One  of  the  first  mutants 
that  appeared,  viz.,  white  eyes,  has  appeared  anew  in 
our  cultures  about  three  times,  in  cultures  known  to 
be  free  from  it  before  and  not  contaminated.  The  same 
mutant  has  been  found  by  several  other  observers. 
The  eye  color  vermilion  has  appeared  at  least  six 
times;  the  wing  character  called  rudimentary,  five 
times ;  cut  wing  has  been  found  four  times,"  etc. 

7.  THE  ORIGIN  OF  MUTATIONS 

Mutations  may  be  gametic,  zygotic  or  somatic  in 
their  origin.  There  seems  to  be  no  reason  why  muta- 
tion may  not  occur  at  any  stage  in  the  life-cycle  of  an 
organism.  In  the  first  place,  it  may  be  gametic  in 
origin  if  the  onset  is  in  the  germ-cell  before  or  during 
the  maturation  changes  that  prepare  it  for  union  with 


54  GENETICS 

another  germ-cell  (See  Chap.  X).  In  this  instance 
its  effect  may  be  profound  and  patent  upon  the  entire 
development  of  the  individual,  although  if  it  chances 
to  be  relegated  to  an  abortive  polar  cell  during  meiosis 
or  to  an  unmated  spermatozoon  it  will  be  entirely  lost 
at  once.  There  are  no  doubt  many  such  "mute  inglo- 
rious mutations"  (Muller)  that  never  see  the  light 
of  day. 

It  is  furthermore  obvious  that  a  gametic  mutation 
usually  enters  the  organism  concerned  singly,  that  is, 
from  one  parent  only,  and  if  recessive 'in  character 
will  fail  to  put  its  appearance  in  the  somatoplasm 
until  some  subsequent  generation  when  two  hybrids 
from  the  new  stock  each  chance  to  contribute  the  re- 
cessive mutant  character  in  question  to  the  formation 
of  a  new  individual. 

The  appearance  of  such  mutants,  therefore,  unless 
dominant,  must  come  two  or  more  generations  after 
the  mutation  has  taken  place.  The  time  when  a 
gametic  mutation  is  initiated,  consequently,  and  when 
it  manifests  itself  are  by  no  means  necessarily  the  same. 
This  fact  needs  to  be  kept  in  mind  in  considering  the 
evidences  from  experiments  for  the  determining  causes 
of  mutations. 

Perhaps  the  reason  why  mutations  are  more  fre- 
quently reported  in  self-fertilizing  (autogamous) 
plants  than  in  cross-fertilizing  (heterogamous)  ani- 
mals is  because  in  self-fertilizing  organisms  the  inbreed- 
ing necessary  to  bring  about  the  doubling  of  a  single 
character  so  that  it  will  come  into  expression  is  more 
likely  to  occur. 


vw    - 

HERITABLE  DIFFERENCES  65 

Secondly,  the  mutation  may  occur  in  the  fertilized 
egg.  This  is  zygotic  mutation.  In  this  case  the  change 
is  evident  at  once  in  the  resulting  individual  since  the 
developing  individual  is  the  unfolding  of  what  is  pres- 
ent in  the  zygote.  Such  a  mutation,  for  example,  oc- 
curring after  fertilization  and  not  as  the  result  of  a 
combination  or  cross,  is  reported  in  tobacco  by  Hayes 
and  Beinhart.1 

Thirdly,  in  contrast  to  the  two  kinds  of  mutations 
just  described  which,  are  distinctly  germinal  in  origin, 
there  may  be  somatic  mutations  which  fall  directly 
upon  some  individual  somatic  cell  or  tissue  arising  out 
of  the  original  germplasm  and  produce  in  turn  such 
abnormalities  as  "bud  variations,"  chimaeras  and  the 
like.  In  such  a  case  all  the  cells  and  tissues  arising 
from  the  mutant  somatic  cell  will  express  the  mutation 
and  no  others.  Lehmann,  1920,  proposes  the  term 
metaclonosis  for  hereditary  somatic  modifications,  re- 
serving the  term  mutation  for  solely  those  instances 
that  involve  a  change  in  the  genes. 

8.  WHEN  MUTATIONS_DCCUR 

It  has  been  suggested  by  Standfuss  that  species 
may  go  through  the  same  kind  of  a  life-cycle  that  indi- 
viduals do,  only  taking  infinitely  more  time  to  do  it. 
As  shown  in  Figure  11,  they  are  born  of  other  species 
and  enter  the  prodigious  growth  period  of  infancy  and 
youth,  both  of  which  are  characterized  by  much  fluctua- 
tion. With  maturity  they  gradually  become  compara- 
tively stable  until  the  reproductive  period  is  reached, 
Science  XXXIX,  No.  992,  p.  34. 


56 


GENETICS 


when  they  throw  off  their  progeny,  as  on  a  tangent. 
They  finally  pass  into  the  excessively  differentiated 
period  of  old  age,  from  which  there  is  no  recall,  al- 
though they  approach  in  many  features  the  infantile 
condition,  and  end  in  death  or  extinction.  This  cycle  is 
repeatedly  illustrated  by  phylogenetic  lines  of  fossil 

forms  which  have 
long  since  become 
extinct. 

Beecher  has 
pointed  out  that, 
in  paleontolog- 
ical  times  just 
before  they  be- 
c  a  m  e  extinct, 
species  often  un- 
derwent extreme 
Fio.  11. — Diagram  of  the  relation  of  re-  ... 

production  to  the  life-cycle.  specialization    in 

the  form  of  fan- 
tastic shapes,  an  excessive  number  of  spines  or  elabo- 
rate sculpturings  on  the  shells  as  seen  among  the 
ammonites,  belemnites,  and  trilobites,  or  of  gigantic 
size  as  in  the  dinosaurs,  plesiosaurs,  and  theromorphs. 
All  of  these  facts  indicate  a  species-cycle  in  which  these 
abnormal  features  were  the  unmistakable  signs  of  old 
age. 

The  reproductive  period  of  a  species  when  mutants 
are  being  thrown  off,  as  of  an  individual,  may  extend 
over  a  considerable  period  of  the  whole  cycle,  or  it 
may  be  confined  to  a  relatively  small  segment.  It  is 
possible  that  in  the  evening  primrose  deVries  may 


HERITABLE  DIFFERENCES  57 

have  caught  a  plant  passing  through  the  crucial  period 
of  species-reproduction. 

Another  reason  why  so  few  mutations  have  as  yet 
been  seen  may  be  because  the  majority  of  organisms 
are  not,  during  the  short  span  of  human  observation, 
in  the  reproductive  part  of  their  cycles.  When  it  is 
remembered  that  accurate  observation  with  this  object 
in  view  has  extended  over  only  a  brief  period,  insig- 
nificant in  comparison  with  the  vast  geologic  stretches 
of  time  concerned  in  species-building,  the  marvel  is 
that  so  much,  rather  than  so  little,  has  been  seen. 

9.  POSSIBLE  CAUSES  OF  MUTATION 

There  are  at  least  three  avenues  of  approach  to  the 
analysis  of  mutation:  (1)  Anatomical,  depending  upon 
observation  of  its  occurrence  in  nature  and  under 
control;  (2)  Genetical,  consisting  of  the  experimental 
breeding  of  test  cases,  and  (3)  Cytolqgical,  or  the 
microscopic  examination  of  the  germplasm.  It  is  this 
latter  method  that  furnishes  perhaps  the  most  hope  of 
gaining  some  insight  into  the  fundamental  causes  under- 
lying the  phenomena  of  mutation. 

No  doubt  the  conclusions  in  this  paragraph  could 
be  better  presented  after  the  consideration  of  the  re- 
maining chapters  of  the  book,  particularly  the  section 
on  the  cellular  basis  of  heredity  (Chaps.  X,  XI  and 
XIII),  but  some  discussion,  nevertheless,  seems  desir- 
able at  this  point,  even  if  it  may  be  necessary  to  return 
and  reread  it  later. 

Babcock  and  Clausen  have  classified  mutations  from 


58  GENETICS 

the  cytological  standpoint  into  two  groups,  viz., 
chromosomal  aberrations  and  factor  mutations.  Chro- 
mosomal aberrations  are  accidents  or  irregularities 
occurring  in  the  nuclear  make-up  of  the  germ- 
cells.  These  aberrations  may  be  of  various  kinds  and 
probably  take  place  during  meiosis  when  the  germ- 
cells  are  going  through  the  preparatory  process  of 
reduction  of  the  chromosomes  which  precedes  the  for- 
mation of  the  fertilized  egg. 

For  example  in  the  unpairing  of  homologous  chro- 
mosomes after  synapsis  it  is  conceivable  that  the  pro- 
cess may  not  be  clean-cut  and  complete  but  that  a  piece 
of  one  chromosome  may  adhere  to  its  mate  thus 
changing  its  size  and  composition.  Or  again,  a  frag- 
ment of  a  chromosome,  during  the  complicated  elimi- 
nation performances  accompanying  the  marriage  cere- 
mony of  germ-cells,  may  be  shuffled  out  and  lost,  thus 
creating  a  deficient  chromosome.  Such  accidents  to  the 
germ-cells  would  be  reflected  in  all  the  subsequent 
mitotic  divisions  of  the  somatic  cells  derived  there- 
from and  a  mutation  would  be  the  result.  At  any  rate 
an  examination  of  the  nuclear  structure  of  mutants 
frequently  reveals  chromosomal  irregularities  so  that 
an  unmistakable  relation  between  the  two  phenomena 
undoubtedly  exists. 

Another  irregularity  that  occurs  is  an  unequal 
migration  of  the  chromosomes  to  the  poles  of  a  germ- 
cell  during  the  reduction  division,  which,  of  course, 
results  in  a  cell  progeny  of  mature  gametes  having 
a  number  of  chromosomes  unlike  the  number  in  the 
normal  gametes.  This  appears  to  be  the  reason  for 


HERITABLE  DIFFERENCES  59 

the  mutation,  (Enothera  lata,  which  has  15  chromo- 
somes instead  of  14,  the  typical  number  for  0. 
lamarckiana  from  which  it  sprang.  What  occurs  in 
the  formation  of  this  mutation  is  that  for  some  reason 
0.  lamarckiana  during  reduction  division  instead  of 
dividing  as  usual  into  7-7  makes  the  unequal  division 
of  6-8,  a  phenomenon  known  as  non-dis junction 
(Bridges).  When  this  8-chromosome  gamete  joins 
with  a  normal  7-chromosome  gamete  the  new  mutant 
number  of  15,  characteristic  of  O.  lata,  is  the  result. 

Gates  and  others  in  their  extensive  cytological 
studies  on  (Enothera  mutants,  have  found  not  only  15 
chromosomes  instead  of  14  but  also,  associated  with 
various  other  mutations,  the  abnormal  numbers  of  16, 
20,  22,  23,  24,  27,  28,  29,  and  30. 

(Enothera  gigas  is  a  mutant  in  which  28  chromo- 
somes, or  twice  the  normal  number,  appear  and, 
moreover,  these  chromosomes  represent  actually  twice 
the  original  amount  of  chromatin  material.  "Gigas" 
mutants  have  been  found  in  various  other  forms,  such 
as  the  tomato  (Winkler),  the  jimson  weed  or  Datura 
(Blakeslee  and  Belling),  Primula  (Gregory)  and 
Narcissus  (Stomps),  and  they  are  always  character- 
ized by  a  doubling  of  the  chromosomes.  This  condition 
is  termed  tetraploidy  because  it  shows  four  times  the 
gametic  number  of  chromosomes. 

When  a  normal  diploid  Datura  is  crossed  with  a  tet- 
raploid  gigas  individual,  a  triploid  mutant  results 
with  a  different  constellation  of  somatic  characters  so 
that  the  best  of  evidence  is  now  at  hand  that  one 
category  of  mutations,  at  least,  that  of  chromosomal 


60  GENETICS 

aberrations,  is  dependent  upon,  or  associated  with, 
abnormal  quantitative  differences  in  the  chromosomes. 

The  other  category  of  mutations,  factor  mutations, 
is  qualitative  and  concerns  the  character  of  hereditary 
units  or  genes  rather  than  quantitative  groups  of  these 
genes  as  they  are  assembled  in  the  chromosomes. 

Whatever  it  is  that  causes  the  character  of  a  gene 
to  change  in  quality,  with  the  resultant  expression  in 
the  somatoplasm,  is  still  apparently  beyond  the  pale 
of  scientific  proof.  Some  investigators  find  satisfac- 
tion in  assigning  external  environmental  causes  to  the 
solution  of  the  problem  while  others  prefer  to  conceal 
their  ignorance  under  the  blanket  of  "internal  causes," 
whatever  these  may  be.  At  least  it  is  reasonable  to 
say  when  a  new  variety  appears  suddenly  in  a  bottle 
full  of  flies  or  in  a  field  of  plants  in  the  same  environ- 
ment with  all  of  its  unmodified  fellows,  that  mutation 
can  arise  somehow  without  outside  interference. 

The  wild  jungle  fowl  presents  a  large  and  useful 
series  of  mutations  which  have  cropped  out  in  poultry 
under  the  spell  of  domestication  while  the  goose,  on 
the  contrary,  although  domesticated  for  an  equally 
long  period,  has  remained  practically  the  same.  The 
nature  of  the  plastic  hen  must  be  different  from  that 
of  the  more  conservative  goose. 

Meanwhile  the  secret  of  the  real  causes  of  muta- 
tions remains  a  challenge  to  every  geneticist  and  suc- 
cess surely  awaits  some  clever  workman  who  knows 
how  to  use  skilfully  the  indispensable  tools  of  obser- 
vation and  experimentation. 

The  bearing  of  the  whole  matter  of  mutation  upon 


HERITABLE  DIFFERENCES  61 

heredity  lies  in  the  fact  that,  contrary  to  Darwin's 
belief,  it  is  apparently  mutations,  and  not  fluctuations 
or  "modifications,'*  that  make  up  heritable  variations. 
If  this  supposition  proves  to  be  true,  mutations  furnish 
the  essential  material  in  the  study  of  heredity.  Conse- 
quently, whatever  knowledge  we  may  gain  of  them  has 
a  direct  relation  to  the  entire  problem  of  genetics. 


CHAPTER  IV 

THE  INHERITANCE  OF  ACQUIRED 
CHARACTERS 

1.  SUMMARY  OF  PRECEDING  CHAPTERS 

HEREDITARY  resemblance  is  due  to  the  derivation  of 
offspring  from  the  same  stock  as  the  parent,  and 
successive  generations,  therefore,  are  simply  periodic 
expressions  of  the  same  continuous  stream  of  germ- 
plasm. 

Perfect  inheritance,  or  uniformity  of  generations, 
does  not  exist,  since  variations  always  occur  in  suc- 
cessive generations.  It  is  upon  these  variations  that 
evolution  depends.  Without  them  there  would  be  no 
change  of  type  and  consequently  no  possibility  of 
evolutionary  advance. 

Some  variations  are  fluctuating  or  continuous  in 
character  and  may  be  detected  and  analyzed  by  sta- 
tistical methods,  while  others  are  mutations,  or  dis- 
continuous variations,  representing  qualitative  differ- 
ences which  do  not  lend  themselves  readily  to  statistical 
analysis. 

Mutations  are  more  common  than  was  formerly 
believed,  and  since  they  are  germinal  rather  than 
somatic  in  character,  they  play  an  important  role  in 
heredity. 

62 


ACQUIRED  CHARACTERS  63 

2.  THE  BEARING  OF  THIS  CHAPTER  UPON  GENETICS 

Only  those  variations  which  reappear  in  succeeding 
generations  have  to  do  with  heredity.  Hence  it  be- 
comes important  to  inquire  as  to  what  kind  of  varia- 
tions actually  reappear.  Can  variations  that  are  not 
inborn,  but  which  are  acquired  during  the  lifetime  of 
the  individual,  be  inherited?  Does  the  experience  of 
the  parent  become  a  direct  part  of  the  child's  heritage, 
or  can  the  environment  of  the  one  enter  in  any  way 
into  the  heredity  of  the  other?  Can  changes  wrought 
in  the  somatoplasm  be  so  impressed  upon  the  germ- 
plasm  as  to  change  it  in  such  a  way  that  it,  in  turn, 
will  give  rise  to  similarly  modified  somatoplasm  in  the 
next  generation?  To  use  Shakespeare's  antithesis,  can 
nurture  as  well  as  nature  be  transmitted?  As  Conklin 
says :  "Few  questions  have  been  discussed  so  fully  and 
so  fruitlessly  as  this." 

In  answering  these  questions  we  are  of  course  con- 
cerned solely  with  biological  inheritance  and  not  at  all 
with  those  extra-biological  accumulations  in  the  way 
of  arts,  literature,  tradition,  invention,  and  the  like 
which  constitute  civilization  and  which  make  us  the 
"heirs  of  the  ages."  Such  benefits  are  entailed  upon 
us  much  in  the  same  way  as  property  is  "inherited," 
but  they  form  no  part  of  the  personal  biological 
heritage  into  which  we  are  now  inquiring. 

3.  THE  IMPORTANCE  OF  THE  QUESTION 

This  inquiry  concerning  the  inheritance  of  acquired 
characters,  which  Professor  Brooks  has  called  "the 


64  GENETICS 

interminable  question,"  is  not  simply  an  academic 
matter.  Its  solution  is  of  vital  importance  from  sev- 
eral viewpoints.  For  breeders,  who  are  trying  to 
maintain  or  improve  particular  strains  of  animals  or 
plants ;  for  physicians,  who,  in  fighting  disease,  are 
honestly  seeking  to  substitute  an  ounce  of  prevention 
for  a  pound  of  cure ;  for  sociologists  and  philanthro- 
pists, who  have  at  heart  the  permanent  bettering  of 
human  conditions;  for  educators,  who  cherish  hopes 
that  their  life-work  of  unfolding  the  youthful  mind 
may  prove  cumulative  and  lasting  rather  than  tran 
sitory;  for  religious  workers,  who  want  their  faith 
strengthened  that  conquests  in  character-building  may 
outreach  the  individual  and  so  enrich  the  race;  for 
parents,  who  entertain  hopes  that  their  own  efforts 
may  give  their  children  a  better  biological  start  in 
life, — for  all  these  and  many  more,  it  is  important  to 
know  the  answer  to  the  question:  Can  acquired 
characters  be  inherited? 


4.  AN  HISTORICAL  SKETCH  OF  OPINION 

That  the  personal  accumulations  of  a  lifetime  are 
heritable  was  generally  believed  throughout  the  credu- 
lous ages.  A  century  ago  Lamarck  made  this  idea 
the  corner-stone  of  his  theory  of  evolution.  It  was  all 
very  simple.  The  reason  evolution  occurs  in  nature 
is  because  individual  acquirements  are  being  continually 
added  to  the  onflowing  stream  of  living  forms.  This 
cumulation  of  characters  indeed  is  evolution.  How 
else  can  the  present  stage  of  adaptation  of  organisms 


ACQUIRED  CHARACTERS  65 

to  their  several  niches  in  nature  be  explained  save  by 
seeing  in  it  the  final  results  of  generations  of  gradu- 
ally inherited  adaptations? 

Darwin  also  believed  in  the  inheritance  of  acquired 
characters,  although  he  differed  from  Lamarck  with 
respect  to  how  such  characters  are  acquired. 

Francis  Galton  in  1875  was  one  of  the  first  to  ex- 
press skepticism  regarding  this  generally  accepted 
belief,  but  the  man  who,  in  a  masterly  manner,  focused 
the  growing  doubt,  and  who  did  more  than  any  other 
to  inspire  thought  and  investigation  upon  the  subject, 
was  August  Weismann,  who  conspicuously  bore  the 
torch  of  genetics  between  1880  and  1890.  Weismann 
made  the  issue  so  clear  that  the  heritability  of  acquired 
characters  became  the  parting  of  the  ways  which 
divided  biologists  into  the  two  camps  of  Neo-Lamarck- 
ians  who  affirm,  and  Neo-Danmnians  who  deny,  such 
inheritance.  His  conclusions,  which  are  the  natural 
outgrowth  of  the  theory  of  the  "continuity  of  the 
germplasm,"  were  based,  however,  upon  logical  rather 
than  upon  experimental  grounds. 

Comparative  anatomists  and  paleontologists,  who 
are  accustomed  to  work  from  results  back  to  their 
causes,  are  frequently  inclined  to  look  favorably  upon 
the  inheritance  of  acquired  characters  while,  on  the 
other  hand,  geneticists  and  embryologists,  representing 
the  two  lines  of  study  which  furnish  the  most  imme- 
diate approach  to  this  problem,  are  well-nigh  agreed 
that  acquired  characters  are  not  inherited.  Experi- 
ment from  cause  to  result  is  undoubtedly  the  best  cri- 
terion for  if  the  question  could  be  decided  by  a  vote 


66  GENETICS 

or  by  an  expression  of  opinion,  the  result  would  be 
doubtful,  since  each  column  contains  the  names  of  men 
whose  scientific  accomplishments  entitle  them  to  a 
respectful  hearing.  But  just  what  are  the  facts  of 
the  case? 

5.  CONFUSION  IN  DEFINITIONS 

The  source  of  much  of  the  lack  of  agreement  in 
this  controversy  lies  in  the  definition  of  what  con- 
stitutes an  "acquired  character."  One  is  reminded 
of  the  two  knights  who  fought  so  bitterly  over  the 
color  of  a  shield,  one  maintaining  that  it  was  red, 
the  other  that  it  was  black.  So  they  hacked  away 
at  each  other,  as  all  good  knights  should  do  in  the 
defense  of  the  truth,  until  they  both  fell  -down  dead 
beside  the  shield  which  was  black  on  one  side  and 
red  on  the  other. 

Of  course  actual  characters  are  never  inherited,  but 
only  the  determiners  or  potentialities  which  regulate 
the  way  in  which  the  organism  reacts  to  its  environ- 
ment with  respect  to  the  characters  in  question.  Reid 
has  pointed  out  that  in  one  sense  every  adult  charac- 
ter is  "acquired"  because  it  has  no  expression  at  first. 
For  instance,  there  is  no  beard  on  the  face  of  a  male 
infant,  but  one  will  presumably  be  "acquired"  later 
on  in  the  life-cycle  due  to  a  heritable  and  not  to  an 
environmental  cause. 

It  is  plain  that  every  new  character  which  repre- 
sents a  forward  evolutionary  step  must  have  been 
"acquired,"  or  added,  sometime  and  somewhere,  else 
it  would  not  be  present,  as  there  is  evidence  that  it  is. 


ACQUIRED  CHARACTERS  67 

Perhaps  the  question,  as  Montgomery  has  suggested, 
ought  to  be  changed  to  read:  "What  kinds  of  acquired 
characters  are  inherited?"  It  is  obvious  that  discus- 
sion is  futile  until  a  common  denominator  in  the  shape 
of  a  definition  of  acquired  characters  shall  be  accepted. 

6.  WEISMANN'S  CONCEPTION  OF  ACQUIRED 
CHARACTERS 

Weismann  defines  an  acquired  character  as  any 
somatic  modification  that  does  not  have  its  origm  in 
the  germplasm. 

Of  course  those  somatic  modifications  which  are 
phases  of  the  developing  individual,  such  as  the 
acquisition  of  a  deeper  voice  at  puberty  or  the  substi- 
tution of  the  permanent  dentition  for  the  milk-teeth, 
are  somatic  variations  which  have  their  rise  and  con- 
trol in  the  germplasm  and  consequently  cannot  prop- 
erly be  included  under  the  head  of  acquired  characters. 

Examples  of  acquired  characters  in  the  Weisman- 
nian  sense  are  mutilations,  the  effects  of  environment, 
the  results  of  function  as  in  the  use  or  disuse  of  certain 
organs,  and  such  diseases  as  may  be  due  either  to  in- 
vading bacteria  or  to  the  neglect  or  abuse  of  the  bodily 
mechanism. 

7.  THE  DISTINCTION  BETWEEN  GERMINAL  AND 
SOMATIC  CHARACTERS 

Redfield  has  thrown  light  on  the  classification  of 
the  characters  which  make  up  the  individual  by  quot- 
ing the  familiar  lines: — 


68  GENETICS 

"Some  are  born  great, 
Some  achieve  greatness, 
Some  have  greatness  thrust  upon  them." 

"Born"  characters  are  constitutional,  having  their 
origin  in  the  germplasm  itself.  They  are  never 
Weismannian  acquired  characters  and  may  be  illus- 
trated by  eye-color,  mental  disposition,  or  facial  fea- 
tures. Lightning  calculators  and  musical  prodigies 
may  have  their  gifts  developed  and  enlarged,  but  the 
fact  that  their  talent  is  nevertheless  an  unmistakable 
gift,  and  not  an  acquisition,  remains  true. 

"Achieved"  characters  are  functional  and  are 
gained  by  exercise.  Many  things  are  achieved,  how- 
ever, which  are  not  acquired  characters,  as,  for  in- 
stance, wealth,  reputation,  or  an  education.  Not  any 
of  these  are  biological  characters,  and  therefore  we 
are  not  concerned  with  them  in  this  connection,  al- 
though in  the  case  of  education  it  should  be  noticed 
that  the  mental  exercise  necessary  to  bring  about  a 
trained  mind,  if  not  the  subject  matter  of  the  educa- 
tion itself,  is  distinctly  an  acquired  character  of  the 
"achieved"  type. 

"Thrust"  characters  are  the  results  of  environment. 
They  are  acquired  without  functional  activity  on  the 
part  of  the  organism  and  usually  in  spite  of  anything 
the  organism  can  do  to  prevent.  Sometimes  these 
characters  are  thrust  upon  the  individuals  before  birth, 
as  in  the  case  of  blindness  caused  by  parental  gonor- 
rhoea or  tuberculosis  arising  from  uterine  infection, 
in  which  case  they  are  termed  congenital  characters. 

Congenital  or  prenatal  characters,  however,  are  in 


ACQUIRED  CHARACTERS  69 

no  way  the  same  as  germinal  characters,  for  they  fall 
just  as  truly  into  the  category  of  acquired  variations 
as  do  those  which  make  their  appearance  in  later  life. 

8.  WHAT  VARIATIONS  REAPPEAR? 

Returning  now  to  Montgomery's  question, — "What 
kinds  of  acquired  characters  are  inherited?'* — it  is 
apparent  that  only  the  "born"  ones  can  be,  which  have 
their  roots  in  the  germplasm  whence  the  new  individual 
arises,  and  that  "achievements"  and  "thrusts,"  in 
order  to  reappear  in  the  succeeding  generation,  can 
do  so  only  by  first  becoming  incorporated  in  the  germ- 
plasm. 

Any  character  that  is  not  acquired  must  have  been 
present  in  the  germplasm  from  which  the  organism 
arose,  as  there  is  no  transfer  of  characters  between 
organisms  except  through  the  germ-cells.  Thus  it  is 
evident  that  the  only  inherited  acquisitions  are  those 
which,  either  primarily  or  .secondarily,  bring  about 
variation  in  the  germplasm.  Such  temporary  acquisi- 
tions as  a  coat  of  tan  or  a  display  of  freckles  do  not 
impress  the  germplasm,  for  when  the  cause  that  incites 
their  appearance  is  removed,  they  soon  vanish. 

9.  How  MAY  GERMPLASM  ACQUIRE  NEW  CHARACTERS? 

In  addition  to  mutation  considered  in  the  last  chap- 
ter, various  sorts  of  rearrangement  in  the  germplasm 
may  present  something  different. 

First  may  be  mentioned  the  "amphimixis"  of  Weis- 
mann,  that  is,  the  mixture  of  two  nearly  related  strains 


70  GENETICS 

of  germplasm  in  sexual  reproduction  within  a  species, 
and  secondly,  the  mixture  of  two  more  remotely  related 
strains  resulting  in  hybridization.  In  either  case  the 
strain  of  germplasm  undergoes  a  shake-up  that  may 
result  at  least  in  new  combinations  of  characters,  if 
not  in  the  production  of  entirely  new  characters.  This 
recombination  of  characters  in  amphimixis  and 
hybridization  will  receive  further  attention  in  a  later 
chapter. 

The  fact  that  successive  parthenogenetic  genera- 
tions, in  which  amphimixis  does  not  of  course  occur, 
may  show  a  larger  degree  of  variability  than  sexually 
produced  generations,  indicates  that  amphimixis  in 
itself  is  by  no  means  sufficient  to  account  for  all  kinds 
of  variations. 

It  is  conceivable  that  the  external  factors  that  act 
upon  the  germplasm  may  be  grouped  into  three  classes : 
— first,  external  factors  acting  upon  the  somatoplasm 
and  then  through  the  agency  of  the  somatoplasm  af- 
fecting the  germplasm  ("somatic  induction"  of  Detto 
or  "pangenesis"  of  Darwin)  ;  second,  external  factors 
acting  directly  upon  the  germplasm  and  the  somato- 
plasm at  the  same  time  ("parallel  induction"  of 
Detto)  ;  and  third,  external  factors  acting  upon  the 
germplasm  without  necessarily  at  the  same  time  hav- 
ing any  effect  upon  the  somatoplasm. 

Many  instances  of  direct  influence  of  external  fac- 
tors upon  germplasm  are  known  in  biological  litera- 
ture, and  these  have  led  to  some  of  the  misunderstand- 
ings concerning  the  "interminable  question"  of  the  in- 
heritance of  acquired  characters.  Sitkowski  fed  the 
caterpillars  of  the  moth  Tineola  biselliella  with  an 


ACQUIRED  CHARACTERS  71 

aniline  dye  (Sudan  red  III),  obtaining  therefrom,  in- 
stead of  normal  whitish  ones,  moths  that  laid  col- 
ored eggs,  and  these  in  turn  hatched  into  caterpillars 
still  tinged  with  the  color  of  the  red  dye.  Riddle,  with 
guinea-pigs,  and  Gage,  with  poultry,  obtained  quite 
similar  results.  This  case  of  apparent  parallel  induc- 
tion, however,  is  not  a  matter  of  inheritance  at  all, 
since  the  germinal  substance  itself  was  not  involved, 
but  of  animals  who  got  their  red  color  directly  from 
external  sources  while  they  were  eggs  within  the  moth- 
er's body. 

10.  WEISMANN'S  REASONS  FOE  DOUBTING  THE  INHER- 
ITANCE OF  ACQUIRED  CHARACTERS 

Weismann's  reasons  for  questioning  the  popularly 
accepted  view  that  acquired  characters  are  inherited 
may  be  briefly  stated  as  follows : — 

First,  there  is  no  known  mechanism  whereby  somatic 
characters  may  be  transferred  to  the  germ-cells. 

Second,  the  evidence  that  such  a  transfer  actually 
does  occur  is  inconclusive  and  unsatisfactory. 

Third,  the  theory  of  the  continuity  of  the  germ- 
plasm  is  sufficient  to  account  for  the  facts  of  heredity 
without  assuming  the  inheritance  of  acquired  somatic 
characters. 

Let  us  examine  these  three  statements  a  little  more 
closely. 

A.    NO  KNOWN  MECHANISM  FOR  IMPRESSING  THE 
GERMPLASM  WITH  SOMATIC  ACQUISITIONS 

Each  germ-cell  remains  an  independent  unit  and 
does  not  participate  in  the  activities  of  the  body  but 


72  GENETICS 

lies  within  the  body  like  a  commensal  or  parasite.  It 
is  hard  to  see,  therefore,  how  a  germ-cell  can  be 
changed  except  in  a  general  nutritive  way  which  is 
quite  different  from  a  change  in  character  of  any 
hereditary  significance. 

The  somatoplasm  is  something  that  has  traveled 
out  from  the  original  fundamental  germplasm  along 
the  paths  of  differentiation  and  elaboration.  The  more 
complex  the  body-cells  become,  that  is,  the  more  suc- 
cessive modifications  they  undergo,  the  more  difficult 
it  is  for  these  somatic  cells  to  return  to  their  original 
primitive  germinal  estate. 

In  many  lower  forms  of  life  where  cell  elaboration 
is  not  so  great,  a  part  lost  by  amputation  is  often 
regenerated,  but  this  process  is  not  possible  in  higher 
forms  where  the  parts  represent  cell  complexes  too 
hopelessly  differentiated  to  begin  anew  the  unfolding 
sequences  of  their  elaboration.  This  difficulty  was 
a  very  real  one  in  the  mind  of  that  famous  nocturnal 
inquirer  Nicodemus  when  he  asked:  "How  can  a  man 
be  born  when  he  is  old?  Can  he  enter  a  second  time 
into  his  mother's  womb  and  be  born?" 

Not  only  the  development  of  the  race  which  we  call 
evolution,  but  also  the  determination  of  the  individual 
in  heredity,  is  a  chain  of  onward-moving  sequences  like 
the  succession  of  events  in  history.  It  is  hard  to  see 
how  recent  events  can  influence  preceding  events.  It  is 
hard  to  see  how  the  water  that  has  gone  over  the  dam 
can  return  and  affect  the  flow  of  the  river  upstream  in 
any  direct  way.  It  is  likewise  hard  to  see  how  differ- 
entiated somatoplasm,  which  represents  the  end  stage 


ACQUIRED  CHARACTERS  73 

of  a  successive  series  of  modifications,  can  make  any 
definite  impress  upon  the  original  germplasmal  sources 
from  which  it  arose. 

Darwin  felt  this  difficulty  and  presented  with  apolo- 
gies his  provisional  hypothesis  of  pangenesis  in  which 
he  assumed  that  every  bodily  part  sends  contributions 
to  the  germ-cells  in  the  form  of  "gemmules."  These 
gemmules,  or  hypothetical  somatic  delegates,  then 
reconstruct  in  the  germ-cells  the  characters  of  the 
entire  body,  including  acquired  modifications  as  well 
as  all  others,  and  thus  there  is  no  reason  why  acquired 
characters  cannot  readily  be  transmitted.  Unfortu- 
nately there  is  no  tangible  basis  in  fact  for  this 
delightfully  simple  explanation  to  rest  upon.  It  is  a 
theory  assuming  that  all  parental  somatic  cells  take 
part  in  the  formation  of  the  new  individual,  hence  it 
was  called  "pangenesis,"  or  origin  from  all. 

Nothing  we  have  subsequently  learned  of  minute 
cell  structure  favors  this  hypothesis,  while  many  facts 
go  quite  against  it.  Moreover,  it  is  directly  opposed 
to  the  theory  of  the  continuity  of  germplasm  so  con- 
vincingly set  forth  later  on  by  Weismann.  Darwin 
indeed  advanced  it  only  in  the  most  tentative  way, 
being  entirely  ready  to  see  it  abandoned  at  any  time 
for  something  better.  It  at  least  performed  one  valu- 
able service  to  science,  namely,  that  of  demonstrating 
how  far  investigators  were  from  an  adequate  concep- 
tion of  any  means  by  which  somatic  modifications  might 
become  incorporated  in  the  germ-cells. 

We  must  acknowledge,  however,  with  Lloyd  Morgan 
that  the  fact  that  a  mechanism  for  the  transfer  of 


74  GENETICS 

somatic  characters  to  the  germ-cells  has  not  been  dis- 
covered, is  not  proof  that  such  a  mechanism  does  net 
exist.  It  may  simply  be  beyond  our  present  powers  of 
penetration. 


B.    EVIDENCE    FOR    TRANSMISSION    OF    ACQUIRED 
CHARACTERS    INCONCLUSIVE 

The  evidence  for  the  inheritance  of  acquired  charac- 
ters was,  for  a  long  time,  taken  for  granted.  This 
theory  was  the  most  obvious  explanation  of  many  facts 
and  so  was  accepted  without  question.  An  obvious 
interpretation,  however,  is  not  always  the  correct  one. 
The  sun  appears  to  go  around  the  earth,  but  astrono- 
mers assure  us  that  it  does  not. 

When  Weismann  began  to  sift  the  evidence  for  the 
inheritance  of  acquired  characters,  he  found  that  it 
was  largely  based  upon  opinion  rather  than  fact,  much 
like  the  popular  belief  with  regard  to  the  causation  of 
warts  by  handling  toads. 

The  supposed  evidence  for  the  inheritance  of  ac- 
quired characters  falls  chiefly  into  the  following  cate- 
gories : — 

a.  Mutilations; 

b.  Environmental  effects ; . 

c.  The  effects  of  use  or  disuse;    . 

d.  The  transmission  of  disease  ;• 

e.  Immunity; 

f.  Prenatal  influences. 


ACQUIRED  CHARACTERS  75 

a.  Mutilations 

It  is  fortunate  that  the  sons  of  warriors  do  not 
inherit  their  fathers'  honorable  scars  of  battle,  else 
we  would  now  be  a  race  of  cripples. 

The  feet  of  Chinese  women  of  certain  classes  have 
for  centuries  been  mutilated  into  deformity  by  ban- 
daging, without  the  mutilation  in  any  way  becoming 
an  inherited  character.  The  same  result  is  also  true 
of  tattooing  and  of  circumcision,  the  latter  a  mutila- 
tion practised  from  ancient  times  by  the  Jews  and 
certain  other  Eastern  peoples.  The  progressive  degen- 
eration or  crippling  of  the  little  toe  in  man  has  been 
explained  as  the  inheritance  of  the  cramping  effect 
of  shoes  upon  generations  of  shoe  wearers,  but,  as 
Wiedersheim  has  pointed  out,  the  fact  that  Egyptian 
mummies  show  the  same  crippling  of  the  little  toe  is 
unfavorable  to  this  hypothesis,  for  no  ancient  Egyptian 
could  ever  be  accused  of  wearing  shoes  or  of  having 
had  shoe-wearing  ancestors.  Sheep  and  horses  with 
docked  tails  as  well  as  dogs  with  trimmed  ears  never 
produce  young  having  the  parental  mutilation. 

Weismann's  classic  experiment  with  mice,  an  experi- 
ment subsequently  confirmed  by  others,  is  additional 
negative  evidence  upon  this  same  point.  What  Weis- 
mann  did  was  to  breed  mice  whose  tails  had  been  cut 
off  short  at  birth.  He  continued  this  decaudalization 
through  twenty-two  generations  with  absolutely  no 
effect  upon  the  tail-length  of  the  new-born  mice. 
One  may  see  in  the  catacombs  of  the  Zoologisches 
Institut  at  Freiburg,  filed  carefully  away  on  shelves, 


76  GENETICS 

as  a  "document,"  long  rows  of  labeled  bottles  contain- 
ing the  fifteen  hundred  and  ninety-two  martyrs  to 
science  which  made  up  the  twenty-two  generations  of 
mice  in  this  famous  experiment. 

Blaringhem,  it  is  true,  obtained  mutations  which 
bred  true  from  latent  buds  that  were  forced  into  devel- 
opment following  mutilation  of  normal  buds,  but 
Griffon  has  shown  that  similar  mutations  occur  with- 
out preceding  mutilations  so  that  this,  as  Shull  points 
out,  is  simply  a  case  of  segregation  of  biotypes  already 
present  in  the  mutilated  parent. 

Conklin  has  hit  the  nail  upon  the  head  with  respect 
to  mutilations  by  saying:  "Wooden  legs  are  not  in- 
herited, but  wooden  heads  are." 

b.  Environmental  Effects 

Trees  deformed  by  prevailing  winds,  like  the  willows 
that  line  the  canals  in  Belgium  and  Holland,  or  storm- 
crippled  trees  along  the  exposed  seacoast  are  not 
known  to  produce  a  modified  progeny  when  their  ad- 
verse environmental  conditions  are  removed.  Simi- 
larly, the  persistent  sunburn  of  Englishmen  long  resi- 
dent in  India  does  not  reappear  in  their  children  born 
in  England. 

Sumner  kept  mice  in  a  constant  but  abnormally  high 
temperature  of  26°  C.  with  the  result  that  the  ears, 
tail,  and  feet  grew  noticeably  larger  than  in  control 
animals  kept  in  ordinary  lower  temperatures,  while  at 
the  same  time  the  general  hairiness  of  the  body  de- 
creased. It  should  be  remembered,  however,  that  mice 


ACQUIRED  CHARACTERS  77 

are  mammals  which  pass  through  an  extended  uterine 
existence,  so  that  it  is  easy  to  see  how  the  offspring 
in  this  case  were  subjected  to  the  same  excessive  tern- 
perature  as  the  parents  for  a  period  sufficient  to  amply 
account  for  their  subsequent  variation  when  removed 
to  a  normal  environment. 

Zederbaur  finds  that  the  wayside  weed  Capsella, 
which  in  the  course  of  many  years  has  gradually  crept 
along  the  roadsides  up  into  an  Alpine  habitat  and 
there  "acquired"  Alpine  characters,  upon  being  trans- 
planted to  the  lowlands  retains  its  Alpine  modifications. 
Although  this  case  has  been  cited  as  an  authentic  in- 
stance of  the  inheritance  of  acquired  characters,  is  it 
not  possible  that  the  conquest  of  the  Alps  by  CapseUa 
has  been  due,  in  the  course  of  time,  not  to  the  inherit- 
ance of  acquired  characters  at  all,  but  to  a  gradual 
natural  selection  of  just  those  germinal  variations 
which  best  fitted  it  to  cope  with  Alpine  conditions 
until,  finally,  a  strain  of  germplasm  producing  somato- 
plasm  suitable  to  Alpine  conditions  has  been  isolated 
in  the  form  of  an  elementary  species  derived  from  the 
original  type?  If  this  is  what  has  happened,  of  course 
such  germplasm  would  give  rise  to  Alpine  plants 
whether  individual  plants  grew  to  maturity  near  the 
snow-line  or  in  the  warm  valleys  at  a  lower  altitude. 

Kammerer,  by  reducing  the  water  supply,  succeeded 
in  transforming  Salamandra  maeulosa,  a  salamander 
normally  producing  about  seventy  eggs  which,  when 
hatched  in  water,  become  gill-breathing  tadpoles,  into 
a  salamander  producing  only  two  to  seven  young 
which  are  born  alive  without  gills  and  are  able  to  live 


78  GENETICS 

out  of  water  entirely,  in  damp  situations.  These 
land-adapted  offspring,  moreover,  when  supplied  with 
abundant  water,  produce  in  turn  tadpoles  which  spend 
days  only,  instead  of  months,  in  the  water  undergoing 
their  metamorphosis,  thus  showing  an  apparent  inherit- 
ance of  an  acquired  character. 

It  should  be  pointed  out,  however,  that  in  these  cases 
the  gill-breathing  forms  in  each  instance  represent  a 
case  of  arrested  development.  Axolotl  is  simply  a 
larval  form  of  Amblystoma  which,  under  normal  con- 
ditions of  an  abundant  water  environment  and  high 
temperature,  gets  no  further  in  its  metamorphosis  than 
the  tadpole  stage,  when  it  produces  eggs  and  sperms 
and  finishes  its  life  story.  A  change  in  environment 
simply  permits  the  life-cycle  to  go  on  further.  Chang- 
ing from  gill-breathing  to  lung-breathing  is  not,  there- 
fore, an  acquired  character,  but  a  purely  germinal 
character  that  may  be  either  blocked  or  released  by 
changing  conditions  in  the  environment.  The  phe- 
nomenon is  termed  neotony. 

c.     The  Effects  of  Use  or  Disuse 

The  callosities  on  the  end  of  a  violinist's  left-hand 
fingers  are  acquired  by  use,  but  they  are  not  inherited. 
There  are  callosities  on  the  knees  of  the  wart-hog, 
Phacochcerus,  which  are  also  apparently  the  result  of 
use,  for  these  animals  kneel  as  they  root  for  a  living  in 
the  African  forests,  and  have  done  so  for  untold  gen- 
erations. It  has  been  noticed  that  young  wart-hogs 
as  soon  as  they  are  born  possess  the  callosities,  so  that 


ACQUIRED  CHARACTERS  79 

this  instance  looks  like  one  of  inheritance  of  a  charac- 
ter acquired  through  use  or  exercise. 

The  skin  on  the  soles  of  human  feet  is  thicker  than 
the  skin  elsewhere,  and  by  use  it  becomes  still  thicker. 
This  is  apparently  another  instance  of  the  same  sort. 
The  writer  has  observed,  however,  that  a  cross  sec- 
tion through  the  foot  of  a  "mud  puppy,"  Necturus 
maculatus,  shows  a  much  thickened  sole.  Necturus, 
it  should  be  noted,  is  a  very  primitive  salamander 
living  always  under  water  and  never  using  the  soles 
of  its  feet  in  any  way  to  bear  its  weight,  nor  is  it 
reasonable  to  suppose  that  it  ever  had  any  ancestors 
who  did  so,  for  the  hands  and  feet  of  the  Amphibia 
are  the  most  primitive  and  ancient  hands  and  feet  to 
be  found  in  the  animal  kingdom  without  any  known 
ancestral  types.  The  thickening  of  the  skin  on  the 
sole  of  the  mud  puppy's  feet  must  be  due,  therefore, 
to  germinal  determiners  and  is  in  no  way  an  acqui- 
sition through  use.  The  same  may  also  be  true  of  the 
wart-hog's  knees  and  of  human  soles. 

The  strong  arm,  the  skilled  hand,  and  the  trained 
ear  are  not  inherited.  They  have  always  to  be  re- 
acquired  in  each  succeeding  generation  just  as  surely 
as  the  ability  to  walk,  or  to  read  and  write. 

Herbert  Spencer  has  defined  instinct  as  "inherited 
habit."  But  surely  those  instincts  which  determine  a 
single  isolated  action  during  the  lifetime  of  the  indi- 
vidual, such  as  the  spinning  of  a  peculiar  cocoon,  can- 
not be  the  result  of  habit,  since  habits  are  formed  only 
through  repeated  action. 

Dr.  Hodge,  who  succeeded  in  hatching  tame  quail 


80  GENETICS 

chicks  out  of  "wild"  eggs,  asks  the  pertinent  ques- 
tion: "How  can  a  fear  hatch  out  of  an  egg?"  The 
habit  of  wildness,  particularly  with  precocial  chicks 
like  quails,  may,  under  an  inciting  environment,  be 
very  soon  established  but  it  is  difficult  to  see  how  cau- 
tion, gained  by  the  experience  of  the  parents,  can  find 
its  way  into  the  fertilized  egg.  If,  then,  some  instincts 
require  a  different  explanation  from  that  of  "inherited 
habit,"  may  it  not  be  likely  that  all  instincts  do?  Is 
it  not  better  to  assume  that  the  structure  of  the  germ- 
plasm  determines  a  particular  response  to  a  particular 
stimulus  regardless  of  whether  in  the  past  the  ances- 
tors have  made  a  similar  response  to  a  similar  stimulus  ? 

d.  Transmission  of  Disease 

If  acquired  diseases  were  heritable  we  would  all  have 
been  dead  long  ago.  When  a  son,  whose  father  died 
of  pneumonia,  succumbs  himself  to  pneumonia  after 
an  interval  of  years  there  may  be  no  more  causal 
or  hereditary  connection  between  the  two  events  than 
when  a  second  house  burns  down  on  the  same  site  where 
a  former  house  went  up  in  flames. 

Many  diseases,  like  tuberculosis,  have  their  imme- 
diate cause  in  invading  pathogenic  bacteria.  Bacteria 
themselves  cannot  be  inherited  for  the  reason  that  it 
is  not  possible  for  them  to  become  an  integral  part  of 
the  fertilized  egg  and  thus  cross  the  "hereditary 
bridge"  which  joins  two  generations.  A  general  pre- 
disposition to  bacterial  disease,  that  is,  a  lack  of  re- 
sistance to  bacterial  invasion  due  to  defectiveness  in 


ACQUIRED  CHARACTERS  81 

physical  or  physiological  equipment,  may  be  present 
as  a  combination  of  characters  in  the  germplasm,  or 
an  individual,  as  the  result  of  disease,  may  "ac- 
quire" a  generally  weakened  germplasm  and  so  pro- 
duce a  progeny  exhibiting  general  liability  to  disease; 
but  it  is  doubtful  if  such  a  condition  can  properly  be 
termed  the  inheritance  of  an  acquired  character,  since 
the  particular  definite  disease  in  question  is  not  de- 
monstrably  heritable. 

When  alcoholism  "runs  in  a  family,"  its  reappear- 
ance in  the  son  is  probably  due  to  the  fact  that  he  is 
derived  from  the  same  weak  strain  of  germplasm  as 
his  father.  The  fact  that  the  father  succumbed  to 
the  alcohol  habit  is  not  the  determining  cause  of 
drunkenness  in  the  son.  The  same  thing  that  caused 
the  father  to  become  an  alcoholic,  namely,  weak  germ- 
plasm,  and  not  the  resulting  drunkenness  in  the  parent, 
is  the  causal  factor  for  alcoholism  in  the  son. 

At  the  same  time  it  is  entirely  probable  that  heredi- 
tary alcoholism  may  in  some  cases  arise  through 
"parallel  induction,"  that  is  to  say,  acquired  alco- 
holism may  end  in  the  simultaneous  poisoning  and 
consequent  modification  of  both  the  somatoplasm  and 
germplasm  of  the  parent,  with  the  result  that  the  germ- 
plasm  has  less  resistance  to  alcoholism  in  a  succeeding 
generation.  The  offspring  are  consequently  more 
likely  to  succumb  to  the  disease.  This,  however,  is 
not  the  inheritance  of  an  acquired  character  or  of  a 
definite  somatic  modification. 

When  a  man  of  the  present  generation  has  rheu- 
matic gout,  it  is  a  severe  stretch  both  of  patriotism 


82  GENETICS 

and  of  the  powers  of  heredity  to  trace  the  origin  of 
the  affliction  back  to  a  revolutionary  ancestor  who 
acquired  sciatic  rheumatism  by  sleeping  on  the  ground 
at  Valley  Forge,  yet  this  is  quite  as  direct  as  many 
alleged  instances  of  the  inheritance  of  disease. 

In  the  majority  of  instances,  apparent  cases  of  the 
inheritance  of  disease  are  merely  instances  of  reinfec- 
tion. This  reinfection  of  the  offspring  may  occur  very 
early  in  embryonic  life,  even  in  the  egg,  in  the  case  of 
pebrine  in  silkworms  (Pasteur)  and  in  the  tick  which 
transfers  the  protozoan  parasite  causing  Texas  fever. 
Or  it  may  happen  after  birth,  provided  the  offspring 
are  exposed  to  the  same  environment  as  that  in  which 
the  parent  acquired  the  disease,  but  in  any  case  reinfec- 
tion is  not  heredity. 

e.  Immwrdty  and  the  Effect  of  Drugs 

Ehrlich  subjected  mice  to  increasing  doses  of  ricin 
until  they  became  immune  to  doses  which  are  ordi- 
narily fatal.  When  these  ricin-immune  mice  were  bred 
to  non-immune  mates  the  offspring  in  turn  showed  some 
degree  of  immunity  if  the  immunized  parent  was  a 
female  but  not  if  the  immunized  parent  was  a  male. 
In  other  words,  the  immunity  was  transferred  through 
the  female  only,  where  the  blood  of  the  mother  is  for 
a  considerable  period  during  foetal  life  in  intimate  rela- 
tion with  the  blood  of  the  offspring.  Even  here,  just 
as  in  the  lifetime  of  an  immunized  individual,  the  im- 
munity tended  to  fade  out  after  a  short  time. 

As  a  matter  of  fact  many  of  the  instances  that  have 


ACQUIRED  CHARACTERS  83 

been  advanced  to  show  the  inheritance  of  acquired 
characters  are  simply  transient  hold-over  somatic 
effects  that  have  gained  no  permanent  grip  upon  the 
hereditary  stream  of  germplasm,  and  which  conse- 
quently soon  fade  away. 

In  a  similar  way  the  gradual  acclimatization  of  the 
mold,  Penicillium,  to  a  salt  solution  of  a  density  suffi- 
cient to  cause  its  death  if  placed  in  it  at  once,  has  been 
effected,  and  the  resulting  spores  have  produced  molds 
that  are  able  to  survive  in  the  concentrated  solution. 
Here,  of  course,  the  spores  have  been  acclimatized  as 
well  as  the  parent  plant  and  it  was  to  be  expected  that 
these  spores  would  develop  into  molds  habituated  to 
the  increased  saline  environment.  This,  however,  is 
pseudo-heredity,  for  no  permanent  method  of  response 
has  been  established. 

/.  Prenatal  Influences 

Perhaps  the  most  illogical  and  at  the  same  time  the 
most  widespread  of  all  types  of  supposed  transmis- 
sion of  acquired  characters  are  the  so-called  "maternal 
impressions."  The  prevalence  of  this  superstition  has 
caused  expectant  mothers  untold  needless  misery. 

Popenoe  and  Johnson,  after  an  excellent  and  ex- 
tended discussion  of  the  matter,  conclude  as  follows : — 

"To  recapitulate,  the  facts  are — 

(1)  That  there  is,  before  birth,  no  connection  be- 
tween the  mother  and  child,  by  which  impressions  on 
the  mother's  mind  or  body  could  be  transmitted  to  the 
child's  mind  or  body. 


84  GENETICS 

(2)  That  in  most  cases  the  marks  or  defects  whose 
origin    is    attributed    to    maternal    impression,    must 
necessarily  have  been  complete  long  before  the  incident 
occurred   which   the   mother,    after  the   child's   birth, 
ascribes  as  the  cause. 

(3)  That   these   phenomena   usually   do   not   occur 
when  they  are,  and  by  hypothesis  ought  to  be,  expected. 
The  explanations  are  found  after  the  event,  and  that 
is  regarded  as  causation  which  is  really  coincidence. 

It  is  easily  understandable  that  any  event  which 
makes  such  an  impression  on  the  mother  as  to  affect 
her  health,  might  so  disturb  the  normal  functioning  of 
her  body  that  her  child  would  be  badly  nourished,  or 
even  poisoned.  Such  facts  undoubtedly  form  the  basis 
on  which  the  airy  fabric  of  prenatal  culture  was  reared 
by  those  who  lived  before  the  days  of  scientific  biology." 

C.    THE    GEEMPLASM    THEORY    SUFFICIENT    TO    ACCOUNT 
FOE   THE    FACTS    OF   HEREDITY 

Weismann  holds  that  the  theory  of  the  continuity 
of  the  germplasm,  already  considered  in  a  previous 
chapter,  is  sufficient  in  itself  to  account  for  the  facts 
of  heredity.  Hence  it  is  quite  unnecessary  to  fall  back 
upon  the  inheritance  of  acquired  characters  as  an 
explanation,  since  this  theory  is  at  least  difficult,  if 
not  impossible,  of  satisfactory  proof. 

To  prove  the  inheritance  of  acquired  characters, 
according  to  Weismann  three  things  are  necessary: 
first,  a  particular  somatic  character  must  be  called 


ACQUIRED  CHARACTERS  85 

forth  by  a  known  external  cause;  second,  it  must  be 
something  new  or  different  from  what  was  already 
exhibited  before,  and  not  be  simply  the  reawakening 
of  a  latent  germinal  character;  and  third,  the  same 
particular  character  must  reappear  in  succeeding 
generations  in  the  absence  of  the  original  external 
cause  which  brought  forth  the  character  in  question. 
As  yet  these  conditions  have  not  been  convincingly 
met  in  the  evidence  which  has  been  brought  forward 
in  support  of  the  inheritance  of  acquired  characters. 

11.  THE  COMPARATIVE  INDEPENDENCE  OF  GERM 
AND  SOMA 

The  fact  that  the  germ  is  only  a  pilgrim  stranger 
passing  through  the  homeless  land  of  the  soma  is  well 
brought  out  by  the  critical  ovarian  transplantation 
experiments  of  Castle  and  Phillips  upon  guinea-pigs. 

The  ovaries  of  an  albino  guinea-pig  were  removed 
and  those  of  a  black  guinea-pig  were  grafted  in  their 
place.  After  recovery  from  the  operation  the  animal 
was  mated  with  an  albino  male  three  times  before 
pneumonia  unfortunately  put  an  end  to  this  famous 
experiment.  The  resulting  offspring  were  all  black, 
as  shown  in  Figure  12.  Ordinarily  when  albinos  are 
crossed  they  produce  only  albinos.  It  is  obvious  that 
the  pneumonia  victim  was  not  the  mother  of  the  six 
black  offspring  although  she  bore  them.  "The  conclu- 
sion is  forced  upon  us,"  to  quote  Babcock  and  Clausen's 
comments  on  the  case,  "that  the  egg-cell  during  its 
growth  does  not  change  in  germinal  constitution.  Its 
growth  is  like  the  growth  of  a  parasite  or  of  a  wholly 


86 


GENETICS 


independent  organism ;  what  it  takes  up  serves  as  food ; 
this  is  not  incorporated  merely  in  the  growing  organ- 
ism, it  is  made  over  into  the  same  kind  of  living  sub- 
stance as  composes  the  assimilating  organism." 


FIG.  12. — Diagram  of  ovarian  transplantation  experiment  to 
show  the  influence  of  somatoplasm  upon  germplasm.  Black 
is  dominant  over  albino.  The  ovaries  from  a  black  guinea-pig 
were  engrafted  into  a  female  albino  whose  ovaries  had  been 
removed.  Upon  recovery  this  female  was  crossed  three  times 
with  an  albino  male.  All  the  progeny  were  black.  Data  from 
Castle  and  Phillips. 

12.  ACQUIRED  CHARACTERS  IN  THE  PROTOZOA 

Although  the  problem  of  the  inheritance  of  acquired 
characters  is  much  better  defined  among  the  higher  ani- 


ACQUIRED  CHARACTERS  87 

mals  where  the  distinction  between  the  soma  and  the 
germ  is  more  sharply  cut  than  among  the  lower  ani- 
mals and  plants,  yet,  as  Jennings  points  out,  one  meets 


FIG.  13. — The  behavior  of  an  "acquired  character," — a  spiny  pro- 
jection at  one  end  of  the  body, — in  the  case  of  Parameciwm. 
The  original  individual  is  represented  in  the  center  and  its 
offspring,  which  arise  by  fission,  are  in  successive  circles.  In 
the  fifth  generation  only  one  out  of  32  shows  the  spine. 
Data  from  Jennings. 


the  same  difficulties  in  the  protozoa  as  in  the  metazoa. 
The  difficulty  in  the  inheritance  of  acquired  charac- 
ters is  not  so  much  in  separating  germ  and  soma  as  in 
the  mechanism  of  cell-division.  There  seems  to  be  no 
way  in  which  an  acquisition  located  at  one  end  of  a 


88  GENETICS 

cell  can  overleap  the  barrier  of  cell  division  and  appear 
at  the  other  end  after  mitosis. 

In  his  cultures  Jennings  found  a  Paramecium  with 
an  abnormal  spine  at  one  end.  This  acquisition  was 
handed  on  for  five  generations  before  it  disappeared 
but  never  in  any  generation  did  more  than  one  of  the 
offspring  have  the  spine.  In  other  words,  it  did  not 
become  hereditary  although  it  continually  reappeared 
in  one  individual  in  every  generation.  The  reason  for 
this  will  be  apparent  upon  referring  to  Figure  13.  The 
fission-half  bearing  the  spine  holds  the  same  relation 
to  the  spineless  half  as  soma  to  germ  and  there  is  here 
no  mechanism  for  the  transmission  from  one  half  to 
the  other.  Simple  transmission,  like  the  persistence  of 
the  spine  for  five  generations  of  Paramecium  is  not 
heredity.  In  order  that  a  character  shall  be  really 
inherited,  that  is,  shall  appear  in  more  than  one  of  the 
progeny  and  so  affect  the  race,  it  must  be  produced 
anew  in  each  generation  from  a  germinal  determiner. 
This  is  just  as  true  for  the  protozoa  as  it  is  for  the 
higher  organisms. 

13.  THE  OPPOSITION  TO  WEISMANN 

The  opponents  of  Weismann  point  out,  as  a  weak 
place  in  his  argument,  the  assumption  that  the  germ- 
plasm  is  so  insulated  from  the  somatoplasm  as  not  to 
be  influenced  by  it.  Weismann  assumes,  of  course, 
that  the  germplasm  is  isolated  from  the  somatoplasm 
very  early  in  the  development  of  the  fertilized  egg  into 
an  individual,  and  that  when  once  isolated  it  thereafter 


ACQUIRED  CHARACTERS  89 

takes  no  active  part  in,  nor  is  in  any  way  affected  by, 
the  vicissitudes  through  which  the  somatoplasm,  or  the 
body  itself,  passes.  The  somatoplasm  is  thus  merely 
a  carrier  of  the  germplasm  and  unable  to  affect  the 
character  of  it  any  more  than  a  rubber  hot-water  bag, 
although  capable  of  assuming  a  variety  of  shapes,  can 
affect  the  character  of  the  water  within  it. 

In  opposition  to  this  view  it  is  urged  that  every 
organism  is  a  physiological  as  well  as  a  morphological 
unity,  and  that  cells  entirely  insulated  within  such  a 
unity  would  be  a  physiological  miracle. 

There  is  abundant  evidence  that  germ-cells,  or 
rather  the  hormones  in  the  sexual  organs  producing  the 
germ-cells,  do  affect  the  somatoplasm  under  particular 
conditions,  as,  for  example,  in  cases  of  castration  when 
those  somatic  features  called  "secondary  sexual  char- 
acters" undergo  profound  modification. 

Even  here,  however,  it  must  be  pointed  out  that  it 
is  not  the  germ-cells  themselves  that  are  directly  re- 
sponsible for  the  modifications  which  occur,  but  rather 
the  hormones  of  the  interstitial  gonadal  cells.  A  most 
serious  fly  in  the  Weismannian  ointment  is  due  to  the 
results  of  certain  recent  experiments  by  Guyer  and 
Smith.1  These  ingenious  experimenters  injected  into 
fowls  the  freshly  removed  lenses  of  rabbits'  eyes  that 
had  been  pulped  up  in  Ringer's  solution.  The  fowls 
developed  an  "anti-body"  which  tended  to  dissolve 
and  disintegrate  the  rabbit  lenses.  When  serum 
from  these  fowls  was  in  turn  injected  into  pregnant 
rabbits  the  mother  was  unaffected  but  nine  out  of  sixty- 
*Jour.  Exp.  Zool  III,  1920. 


90  GENETICS 

one  surviving  young  were  born  with  degenerate  eyes. 
The  affected  young  have  carried  the  defect  even  in 
the  male  line  through  eight  generations  without  the 
injection  of  any  more  serum  containing  the  lens  anti- 
body. "The  degenerating  eyes  are  themselves,  directly 
or  indirectly,  originating  anti-bodies  in  the  blood  serum 
of  their  bearers — which  in  turn  affect  the  germ-cells." 
If  these  conclusions  are  substantiated,  the  cardinal 
principle  of  the  inheritance  of  acquired  characters  is 
conceded.  The  end  is  not  yet ! 

14.  VARIOUS  RESULTS  UPON  OFFSPRING  OF  PARENTAL 
ACQUISITIONS 

In  Diagram  14  an  attempt  is  made  to  visualize  the 
various  results  of  parental  acquisitions,  both  somatic 
and  germinal,  upon  the  generations  following. 

It  will  be  noted  that  Case  I,  where  the  soma  of  the 
parent  is  represented  as  determining  the  soma  of  the 
offspring,  is  contrary  to  fact  for  in  sexual  reproduc- 
tion the  offspring  arises  from  the  undifferentiated 
germplasm  of  its  parents. 

The  usual  result  of  a  somatic  modification  is  shown 
in  Case  II. 

Pangenesis,  Case  III,  postulates  a  reversal  of  the 
universal  process  of  differentiation  in  that  it  demands 
a  return  of  the  elaborated  soma  with  the  modifications 
it  has  acquired  during  the  course  of  its  elaboration, 
to  the  primitive  condition  of  the  germ. 

In  Case  IV  the  apparent  inheritance  of  acquired 
characters  is  due  not  to  the  fact  that  the  parental  soma 


ACQUIRED  CHARACTERS 


91 


92  GENETICS 

was  modified,  but  because  at  the  same  time  and  in  the 
same  way  that  the  parental  soma  was  taking  on  a 
modification,  the  germ  was  likewise  modified.  This,  to 
use  the  drug  clerk's  phraseology,  is  "something  just  as 
good"  as  the  inheritance  of  acquired  characters  but 
it  is  not  the  Weismannian  brand. 

Finally,  Case  V  shows  a  true  mutation  which  occurs 
in  the  parental  germplasm  but  does  not  appear  to  the 
light  of  day  until  the  offspring  develops. 

15.  CONCLUSION 

But  even  granting  that  the  somatoplasm  affects  the 
germ-cells,  the  inheritance  of  acquired  characters  is 
by  no  means  thereby  established. 

In  order  to  do  this,  the  precise  acquired  character 
in  question,  which  indirectly  exercised  its  influence 
upon  the  germ,  must  be  redeveloped,  and,  although 
the  germplasm  might  conceivably  receive  an  influence 
from  the  somatoplasm  and  be  affected  by  it  in  a  gen- 
eral way,  it  is  a  different  matter  entirely  to  develop 
anew  the  replica  of  the  character  itself  which  is  sup- 
posed to  have  been  acquired. 

It  will  be  seen  in  subsequent  pages,  under  the  dis- 
cussion of  data  furnished  by  experimental  breeding, 
that  the  weight  of  probability  is  decidedly  against 
the  time-honored  belief  in  the  inheritance  of  acquired 
characters. 


CHAPTER  V 

MENDELISM 
1.  METHODS  OF  STUDYING  HEREDITY 

MODERN  studies  in  heredity  have  been  pursued  princi- 
pally in  three  directions :  first,  by  microscopical  ex- 
amination of  the  germ-cells;  second,,  by  statistical 
consideration  of  data  bearing  upon  heredity ;  and 
third,  by  experimental  breeding  of  animals  and  plants. 
In  the  present  chapter  attention  will  be  directed  to  a 
consideration  of  experimental  breeding  with  reference 
to  hybridization,  that  is,  breeding  from  unlike  parents, 
a  process  which  Jennings  characterizes  by  the  expres- 
sive phrase,  "the  melting-pot  of  cross-breeding." 

2.  THE  MELTING-POT  OF  CROSS-BREEDING 

Hybridization,  or  cross-breeding,  as  analyzed  by 
^Galton  (1888),  results  in  one  of  three  kinds  of  inherit- 
ance, namely,  blending,  alternative,  or  particulate. 

Of  these,  blending  inheritance  may  be  called  ,the 
typical  ''melting-pot"  in  which  contributions  from  the 
two  parents  fuse  into  something  intermediate  and  dif- 
ferent from  that  which  was  present  in  either  parent. 
Galton  illustrated  this  process  by  the  inheritance  of 
human  stature  in  which  a  tall  and  a  short  parent  pro- 

93 


GENETICS 


duce  offspring  intermediate  in  height.  A  more  thorough 
consideration  of  this  type  of  inheritance  will  be  pre- 
sented in  Chapter  VIII. 

By  the  method  of  alternative  inheritance  the  pa- 
rental contributions  do  not  melt  upon  union,  but  re- 
tain their  individuality,  reappearing  intact  in  the  off- 


Charaettristica 
Of  parental 


Blending 


Alternative 


Particulate 


may  produce 


FIG.  15.  —  Three  kinds  of  inheritance  described  by  Galton,  when 
applied  to  a  single  pair  of  characters. 

spring.     In  inheritance  of  human  eye-color,  fo 
ample,  the  offspring  usually  have  eyes  colored  like  -tKtV 
of  one  of  the  parents  when  the  parental  eye-co!0/  5 
unlike  in  the  two  cases,  rather  than  eyes  intermedia 
in  color  between  those  of  both  parents. 

Particulate  inheritance  results  when  the  offspring 
present  a  mosaic  of  the  parental  characters,  that  is, 
when  parts  of  both  the  maternal  and  paternal  charac- 
ters reappear  in  the  offspring  without  losing  their  iden- 


MENDELISM  95 

titles  by  blending  or  without  excluding  one  another. 
Piebald  races  of  mice  arising  from  parents  with  solid 
but  different  colors  have  been  cited  as  illustrations  of 
this  sort  of  inheritance,  although  it  will  be  seen  later 
in  connection  with  the  "factor  hypothesis"  that  another 
interpretation  of  this  phenomenon  is  not  only  possible 
but  probable. 

The  distinctions  between  these  three  categories  of 
inheritance  are  diagrammatically  represented  in  Figure 
15. 

3.  JOHANN  GREGOR  MENDEL 

Our  understanding   of   the  working  of   inheritance 
in  hybridization  we  owe  largely  to  the  unpretentious 
studies  of  an  Austrian  monk,  Johann  Gregor  Mendel, 
who,  although  a  contemporary  of  Darwin,  was  prob- 
ably unknown  to  him.     Bateson  says  of  Mendel:  "Un- 
troubled by  any  itch  to  make  potatoes  larger  or  bread 
heaper  he  set  himself  in  the  quiet  of  a  cloister  garden 
D  find  out  the  laws  of  hybridity,  and  so  struck  a  mine  of 
uth,   inexhaustible   in   brilliancy    and   profit."      For 
Mit  years  Mendel  carried  on  original  experiments  by 
Deeding  peas  and  then  sent  the  results  of  his  work  to  a 
j  jrmer  teacher,  the  celebrated  Karl  Nageli,  of  the  Uni- 
-  vversity  of  Vienna.    At  the  time  Nageli's  head  was  full  of 
other  matters,  so  that  he  failed  to  see  the  significance  of 
lis   old  pupil's   efforts.      However,   in   1866   Mendel's 
results  appeared  in  the  Transactions  of  the  Natural 
'History    Society   of   Briinn,1    an   obscure   publication 

1  Verhandlungen  naturf.  Verein  in  Briinn.  Abhandl.  IV,  1865 
r(which  appeared  in  1866). 


96  GENETICS 


that  reached  hardly  more  than  a  local  public.  Here 
Mendel's  investigations  were  buried,  so  to  speak,  because 
the  time  was  not  ripe  for  a  general  appreciation  or 
evaluation  of  his  work. 

At  that  time  neither  the  chromosome  theory  nor  the 
germplasm  theory  had  been  formulated.  Moreover, 
much  of  our  present  knowledge  of  cell  structure  and 
behavior  was  not  even  in  existence.  Weismann  had  not 
yet  led  out  the  biological  children  of  Israel  through  the 
wilderness  upon  that  notable  pilgrimage  of  fruitful 
controversy  which  occupied  the  last  two  decades  of  the 
nineteenth  century,  and  the  attention  of  the  entire 
thinking  world  was  being  monopolized  by  the  newly 
published  epoch-making  work  of  Charles  Darwin. 

Mendel  died  in  1884,  and  his  work  slumbered  on 
until  it  was   independently   discovered,   almost   simul- 
taneously,  by   three   botanists   whose   researches   had 
been  leading  up  to  conclusions  very  much  like  his  own. 
These  three  men  were  deVries  of  Holland,  von  Tscher 
mak  of  Austria,  and  Correns  of  Germany.  Their  con 
tributions  were  published  only  a  few  months  apart  c 
1900  and  were  closely  followed  by  important  papers 
from    Bateson    in    England,    Cuenot    in    France    and., 
Davenport  and  Castle  in  America,  extending  Mendelisr^ 
to  animals,  with  a  rapidly  increasing  number  from  ot 
biologists  the  world  over.  To-day  the  literature  uj 
this  subject  has  grown  to  be  very  large,  and  the  end 
by  no  means  yet  in  sight. 

Castle  has  well  said:     "Mendel  had  an  analyti 
mind  of  the  first  order  which  enabled  him  to  plan  an 
carry  through  successfully  the  most  original  and  in 
structive  series  of  studies  in  heredity  ever  executed." 


MENDELISM 


97 


lt.  MENDEL'S  EXPERIMENTS  ON  GARDEN  PEAS 

JVhat  Mendel  did  was  to  hybridize  certain  varieties 
garden  peas  and  keep  an  exact  record  of  all  the 
igeny,  in  itself  a  simple  process  but  one  that  had 
;ver  been  faithfully  carried  out  by  any  one. 
"To  Mendel's  foresight  in  arranging  the  conditions 
his  work,  as  much  as  to  his  astuteness  in  interpreting 
e  data,  is  due  his  remarkable  success."     (Morgan.) 
Before  examining  Mendel's  results  it  may  be  well  to 
late  the  difference  between  normal  and  artificial  self- 
:ilization.     Self-fertilization  occurs  when  from  the 
>llen  and  ovule  of  the  same  flower  are  derived  the  two 
imetes  which  uniting  produce  a  zygote  that  develops 
[to  the  seed  and  subsequently  into  the  adult  plant  of 
next  generation.     In  artificially  crossing  normally 
[f-fertilized  flowers  it  is  necessary  to   carefully  re- 
ive the  stamens  from  one  flower  while  its  pollen  is 
ill  immature,  and  later,  at  the  proper  time,  to  transfer 
it  ripe  pollen  from  another  flower. 
[Mendel's  cross-breeding  experiments  on  peas  showed 
;ain  numerical  relations  among  the  progeny  which 
re  rise  to  what  has  come  to  be  rather  indefinitely 
as  "Mendel's  law."    This  law  may  be  temporarily 
ftrmulated  as  follows : — 

B*Vhen  parents  that  are  unlike  with  respect  to  any 
•acter  are  crossed,  the  progeny  of  the  first  genera- 
will  apparently  be  like  one  of  the  parents  with 
to  the  character  in  question.  The  parent 
nresses  its  character  upon  the  offspring  in  this 
manner  is  called  the  dominant.  When,  however,  the 
Jiybrid  offspring  of  this  first  generation  are  in  turn 


ch 


98  GENETICS 

crossed  with  each  other,  they  will  produce  a  mixefl 
progeny,  25  per  cent  of  which  will  be  like  the  dominanB 
grandparent,  25  per  cent  like  the  other  grandparenl 
and  50  per  cent  like  the  parents  resembling  the  doi 
nant  grandparent. 

An  illustration  will  serve  to  make  plain  the  manm 
in  which  this  law  works  out. 

Mendel   found   that   when  peas    of   a    tall   varietj 
were  artificially  crossed  with  those  of  a  dwarf  variety 
all  the  resulting  offspring  were  tall  like  the  first  parenj 
It  made  no  difference  which  parent  was  selected  as  tl 
tall   one.      The   result   was   the   same  in   either   cas 
showing  that  the  character  of  tallness  is  independei 
of  the  character  of  sex. 

When  these  tall  cross-bred  offspring  were  subsj 
quently  crossed  with  each  other,  or  allowed  to  p 
duce  offspring  by  self-fertilization  which  amounts 
the  same  thing,  787  plants  of  the  tall  variety  and  277? 
of  the  dwarf  kind  were  obtained,  making  approximated 
the  proportion  of  3  to  1. 

On  further  breeding  the  dwarf  peas  thus  derivdH 
proved  to  be  pure,  producing  only  dwarf  peas,  whih* 
the  tall  ones  were  of  two  kinds,  one  third  of  thai 
"pure,"  breeding  true  like  their  tall  grandparent,  and 
two  thirds  of  them  "hybrid,"  giving  in  turn  the  PWJ 
portion  of  three  tall  to  one  dwarf  like  their  pareiM 

These  crosses  may  be  expressed  as  follows : — 

Tall,  T,  X  dwarf,  t,  =  tall,  T(t). 

That  is,  tallness  crossed  with  dwarfness  equals  tallness 
with  the  dwarf  character  present  but  latent. 


MENDELISM 


99 


Mendel  termed  the  character,  which  became  apparent 
in  such  a  hybrid,  in  this  case  tallness,  the  dominant, 
and  the  latent  character  which  receded  from  view,  in 
this  instance  dwarfness,  the  recessive. 

The  members  of  such  a  Mendelian  pair  are  termed 
allelomorphs. 

When  now  the  hybrids,  T(t),  were  crossed  together, 
the  result  algebraically  expressed  was  as  follows: — 

T  +  t  (all  possible  egg  characters) 
T  -j-  t  (all  possible  sperm  characters) 

TT  +     Tt 

Tt      +  tt 

TT  +  2T(t)  +  tt 

That  is,  one  of  the  four  possible  cases  was  dwarf,  1 1, 
in  character  and  the  other  three  were  apparently  tall, 
although  only  one  out  of 
the  three  was  pure  tall, 
TT,  while  the  remaining 
two  were  tall  with  the 
dwarf  character  latent, 
T(t). 

The  same  thing  may  be 
expressed  more  graph- 
ically by  the  checkerboard 
plan,  which  Punnett  sug- 
gested (Fig.  16).  Each 


Male  Gametes      ^ 

L      f 

W31- 

t 

<§ 

-     tT 

it 

FIG.  16. — Diagram  to  illustrate 
theoretically  the  formation  of 
the  four  possible  zygotes  in 
the  second  filial  generation  of 
a  monohybrid. 


square     of     the     checker- 
board represents  a  zygote 

which,  having  received  a  gamete  from  each  of  the  two 
parents,  may  develop  into  a  possible  offspring.  The 
character  of  the  gametes  of  the  parents  is  shown  out- 


100 


GENETICS 


side  of  these  squares,  while  the  arrows  represent  the 
parental  source  from  which  the  offspring  have  received 
their  hereditary  composition. 

The  essential  feature  of  Mendel's  law  is  briefly  this : 
hereditary  characters  are  visually  independent  units 
which  segregate  out  upon  crossing,  regardless  of  tem- 
porary dominance. 

Mendel  carried  on  further  experiments  with  garden 
peas,  using  other  characters.  He  obtained  practically 
the  same  result  as  in  the  instance  already  given,  for 
the  actual  progeny  in  the  second  generation  of  the 
cross-bred  offspring  figured  up,  as  seen  in  the  table 
below,  very  nearly  to  the  expected  theoretical  ratio  of 
3  to  1. 


CHARACTER 

NUMBER  OF 
DOMINANTS 

NUMBER  OF 
RECESSIVES 

RATIO 

Form  of  seed       .      . 
Color  of  seed  coat  . 
Length  of  stem  .     . 
Color  of  flowers 
Position  of  flowers 
Form  of  pods    . 
Color  of  unripe  pods 

5474  smooth 
6022  yellow 
787  tall 
705  colored 
651  axial 
882  inflated 
428  green 

1850  wrinkled 
2001  green 
277  dwarf 
224  white 
207  terminal 
299  constricted 
152  yellow 

2.96  to 
3.01  to 
2.84  to 
3.15  to 
3.14  to 
2.95  to 
2.82  to 

Total      

14949 

5010 

2.98  to  1 

These  results  have  been  confirmed  by  other  investi- 
gators, for  example  the  yellow-green  seed-color  cross 
has  been  repeated  by  Correns,  Tschermak,  Hurst,  Bate- 
son,  Lock,  Darbishire  and  White,  with  results  totalling 
195,477  in  the  second  generation  of  which  number 
146,802  were  yellow  and  48,675  were  green.  This  is  a 
proportion  of  3,016  to  1. 


MENDELISM  1(H 

5.  SOME  FURTHER  INSTANCES  OF  "MENDEL'S  LAW" 

Since  the  rediscovery  of  Mendel's  law  the  ratio  of 
3  to  1  in  the  second  hybrid  generation  has  been  found 
by  a  number  of  different  investigators  to  be  constant  in 
a  large  array  of  characters  observed  both  in  animals 
and  plants  of  diverse  kinds  when  these  are  cross-bred 
with  reference  to  the  characters  in  question. 

Botanists  have  an  advantage  perhaps  in  this  matter, 
as  they  deal  with  forms  which  usually  produce  a  large 
number  of  offspring  from  a  single  cross,  a  very  desir- 
able condition  in  estimating  ratios.  On  the  other  hand, 
they  are  handicapped  by  being  unable  usually  to  obtain 
more  than  one  generation  in  a  year,  while  zoologists 
may  secure  from  animals  like  rabbits  and  mice  several 
generations  in  a  year,  although  ordinarily  the  number 
of  progeny  is  much  smaller  and  the  ratios  obtained 
have  a  larger  chance  of  error  than  is  the  case  with  the 
more  numerous  plant  offspring. 

Semi-microscopic  animals,  as,  for  example,  the  pom- 
ace fly,  Drosophila,  which  produces  a  large  progeny 
every  two  weeks  or  so,  may  combine  the  general  ad- 
vantages mentioned  for  the  two  groups  of  organisms 
indicated  above,  but  they  have  the  disadvantage  of 
being  so  small  that  the  detection  of  their  distinctive 
phenotypic  characters  is  attended  with  considerable 
technical  difficulty. 

What  the  modern  experimenter  in  genetics  desires  is 
an  organism,  first,  which  possesses  conspicuous  distinc- 
tive somatic  characters,  and,  second,  that  will  come  to 
sexual  maturity  early  and  breed  either  in  captivity  or 


103 


G'ENETICS 


under  cultivation  both  numerously  and  frequently  with 
a  minimum  of  trouble  and  expense. 

The  following  table,  compiled  chiefly  from  Bateson  l 
and  Baur,2  might  easily  be  much  extended.     It  shows 


ORGANISM 

AUTHOR 

Q 

DOMINANT 

RECESSIVE 

Nettles 

Correns 

'03 

Serrated  leaves 

Smooth-margined 

leaves 

Sunflower 

Shull 

'08 

Branched  habit 

Unbranched  habit 

Cotton 

Balls 

'07 

Colored  lint 

White  lint 

Snapdragon 

Baur 

'10 

Red  flowers 

Non-red  flowers 

Wheat 

Biffen 

'05 

Susceptibility 

[mmunity  to  rust 

to  rust 

Tomato 

Price  and 

'08 

Two-celled  fruit 

Many-celled  fruit 

Drinkard 

Maize 

deVries 

'00 

Round,  starchy 

Wrinkled,  sugary 

kernel 

kernel 

Silkworm 

Toyama 

'06 

Yellow  cocoon 

White  cocoon 

Cattle 

Spillman 

'06 

Hornlessness 

Horns 

Pomace  fly 

Morgan 

'10 

Red  eyes 

White  eyes 

Horses 

Bateson 

'07 

Trotting  habit 

Pacing  habit 

Land  snail 

Lang 

'09 

Jnbanded  shell 

Banded  shell 

Mice 

Darbishire 

'02 

formal  habit 

Waltzing  habit 

Guinea-pig 

Castle 

'03 

Short  hair 

Angora  hair 

Canaries 

Bateson  and 

'02 

Crest 

Plain  head 

Saunders 

Poultry 
Man 

Davenport 
Farrabee 

'06 
'05 

:lumplessness 
Brachydactyly 

Long  tail 
formal  joints 

Barley 

von  Tschermak 

'01 

Beardlessness 

Beardedness 

Salamander 

(Ambly- 

stoma) 

Haecker 

'08 

Dark  color 

Light  color 

from  what  diverse  sources  confirmatory  evidence  of  the 
truth  of  Mendel's  law  has  been  derived  within  the 
first  ten  years  of  observation  and  experiment  after  its 
rediscovery. 

1  "Mendel's  Principles  of  Heredity,"   1909. 

a  "Einfiihrung  in  die  experimented  Vererbungslehre,"  1911. 


MENDELISM  103 

6.  THE  CARDINAL,  PRINCIPLE  OF  SEGREGATION 

The  essential  thing  which  Mendel  demonstrated  was 
the  fact  that,  in  certain  cases  at  least,  the  determiners 
of  heredity  derived  from  diverse  parental  sources  may 
unite  in  a  common  stream  of  germplasm  from  which, 
in  subsequent  generations,  they  may  segregate  out  ap- 
parently unmodified  by  having  been  intimately  asso- 
ciated with  eadh  other.  This  law  of  segregation,  or 
"independent  assortment"  as  Morgan  prefers  to  call  it, 
depends  upon  the  conception  that  the  individual  is 
made  up  of  a  bundle  of  unit  characters.  It  may  be 
illustrated  by  the  separate  flowers  picked  from  a  garden 
which,  after  being  made  into  a  nosegay,  may  be  taken 
apart  and  rearranged  without  in  any  way  disturbing 
the  identity  of  the  separate  blossoms. 

The  general  formula  of  segregation  that  covers 
all  cases  of  organisms  cross-bred  with  respect  to  a 
single  character,  that  is,  monohybricfo,  is  given  in 
Figure  17.  , 

The  parents  of  a  hybrid  are  usually  referred  to  as 
the  parental  generation  (P).  The  hybrid  generation 
formed  by  crossing  diverse  characters  'in  parents  is 
designated  as  the  first  filial  generation  (Fj).  The 
offspring  of  Fx  are  F2,  and  so  on. 

Incidentally  this  diagram  hints  how  it  is  possible 
to  derive  a  pure  strain  from  an  impure  (hybrid)  source, 
a  fact  of  immediate  interest  not  only  to  breeders  of  ani- 
mals and  plants  but  also  to  breeders  of  men. 

Such  "extracted"  recessives  or  dominants  will  be  en- 
tirely free  of  the  hybrid  impurity. 


104  GENETICS 


7.  DEFINITIONS 


A   character  which  is   present   in  the  offspring  in 
double  quantity  because  it  was  present  in  both  parents 
is   said  by  Bateson  to   be  homozygous,  while  an  or-' 
ganism  which  is  homozygous  with  respect  to  any  char- 


D  (Dominant) 


DD 

ZD&) 

RR 
\ 

D» 

2  Dp) 

^ 

DD      DD      DD.      2D(R)         RR      RR        RR 
FIG.  17. — General  Mendelian  formula  for  a  monohybrid. 

,-> 

acter  is  called  a  homozygote  so  far  as  that  particular 
character  is  concerned  (DD  or  RR.) 

In  contrast  to  the  homozygous  condition,  an  organ- 
ism is  said  to  be  heterozygous  when  it  derives  the  deter- 
miner of  a  character  from  only  one  parent.  Such 
an  organism  is  described  as  a  heterozygote  with  respect 
to  the  character  in  question  (DR). 

Organisms  that  appear  to  be  alike,  regardless  of 
their  germinal  constitution,  are  said  by  Johannsen  to 
be  identical  phenotyjpically  (DD  and  DR),  while  or- 
ganisms having  identical  germinal  determiners-  are 
said  to  be  genotypically  alike  (DD  and  DD  or  RR 
and  RR). 


MENDELISM  105 

The  word  "genotype"  was  suggested  by  Jbhannsen 
in  honor  of  Darwin  and  his  theory  of  pangenesis,  al- 
though there  are  certain  objections  to  its  use  in  this 
connection  for  the  reason  that  systematists  have 
already  appropriated  it  in  a  different  sense.  As  here 
used  it  signifies  "the  fundamental  hereditary  constitu- 
tion or  combination  of  genes  of  an  organism"  (Shull). 

8.  THE   IDENTIFICATION   OF  A  HETEROZYGOTE 

"Homozygote"  and  "heterozygote"  are  terms  then 
descriptive  solely  of  the  genotypical  constitution  of 
organisms,  and,  as  has  been  said,  it  is  not  always  pos- 
sible to  distinguish  one  from  the  other  by  inspection. 
The  only  sure  way  to  identify  a  heterozygote  is  by 
breeding  to  a  recessive  and  observing  the  kind  of  off- 
spring produced. 

Peas  of  the  formulae  TT  and  T(t)9  for  example, 
both  look  alike,  since  a  single  determiner  for  the  tall 
character,  T,  is  sufficient  to  produce  complete  tallness. 
When,  however,  these  two  kinds  of  tall  peas  are  each 
bred  to  a  recessive  dwarf  pea,  of  the  formula  tt,  the 
progeny  will  differ  distinctly  in  the  two  cases  as  fol- 
lows :  — 

Case    I.     T  +  T  X  t  +  t  =  100  per  cent  T(t). 

Case  II.     T      t  X  *  +  *  =  50  per  cent  T(t)  +  $0  per  cent  tt.  - 


That  is,  if  the  dominant  to  be  tested  is  homozygous 
(Case  I),  the  entire  progeny  will  exhibit  the  dominant 
character,  but  if  the  dominant  to  be  tested  is  heterozy- 
gous (Case  II),  then  only  one  half  of  the  progeny  will 
show  the  character  in  question. 


106  GENETICS 

Sometimes  when  dominance  is  not  pronounced  it  is 
possible  to  distinguish  the  heterozygote  dominant  from 
the  homozygote  dominant.  Correns  has  described  an 
excellent  instance  of  this  type.  When  plants  of  a 
white-flowering  race  of  the  four-o'clock,  MirabUis 
jalapa,  are  crossed  with  those  of  a  red-flowering  race, 
all  the  offspring  in  the  first  filial  generation,  unlike 
either  parent,  exhibit  rose-colored  flowers.  When,  how- 
ever, these  rose-colored  flowers  are  crossed  with  each 
other,  they  produce  red,  rose,  and  white  in  the  Men- 
delian  ratio  of  1:2:1;  that  is,  three  colored  to  one 
white.  The  red-flowering  race  thus  proves  to  be 
homozygous  and  the  rose-flowering  race  heterozygous. 
Here  color  dominates  the  absence  of  color,  or  white, 
but  the  degree  of  the  color  depends  upon  whether  the 
dose  of  pigment  is  double,  from  both  parents,  or  single, 
from  only  one  parent. 

9.  THE  "PRESENCE  OR  ABSENCE"  HYPOTHESIS 

In  place  of  Mendel's  conception  that  every  dominant 
character  is  paired  with  a  recessive  alternative  or 
allelomorph^  there  has  been  proposed  the  presence  or 
absence  hypothesis  which  was  first  suggested  by  Correns 
but  later  logically  worked  out  by  others,  particularly  by 
Hurst,  Bateson,  and  Shull.  According  to  this  inter- 
pretation, a  determiner  for  any  character  either  is, 
or  is  not,  present.  When  it  is  present  in  two  parents, 
then  the  offspring  receive  a  double,  or  duplex,  "dose," 
to  use  Hurst's  word,  of  the  determiner.  When  it  is 
present  in  only  one  parent,  then  the  offspring  have 


MENDELISM  107 

a  single,  or  simplex,  dose  of  the  character.  When  it 
is  present  in  neither  parent,  it  follows  that  it  will  not 
appear  in  the  offspring.  In  this  case  the  offspring 
are  said  to  fie  nulivplex  with  respect  to  the  character 
in  question.  Take  the  case  of  tall  and  dwarf  peas, 
the  determiner  for  t  alines  s  when  present  produces  tall 
peas,  even  if  it  comes  from  only  one  parent,  but  if  this 
determiner  for  tallness  is  absent  from  both  parents, 
the  offspring  are  nulliplex,  that  is,  the  absence  of  tall- 
ness  results  and  only  dwarf  peas  are  produced. 

The  difference  between  the  presence  or  absence 
theory  and  the  dominant  or  recessive  theory  of  allelo- 
morphs is  that  in  the  former  case  the  "recessive"  char- 
acter has  no  existence  at  all,  while  in  the  latter"  in- 
stance it  is  present,  but  in  a  latent  condition. 

The  reasons  for  and  against  the  presence  or  absence 
interpretation  may  be  more  suitably  considered  later. 


10.  DIHYBRIDS 

So  far  reference  has  been  made  exclusively  to  mono- 
hybrids,  any  two  of  which  are  supposed  to  be  similar 
except  with  respect  to  a  single  unit  character.  Mono- 
hybrids  are  comparatively  simple,  but  when  two  or- 
ganisms are  crossed  which  differ  from  each  other  with 
respect  to  two  different  unit  characters,  the  situation 
becomes  more  complicated. 

Mendel  solved  the  problem  of  dihybrids  by  crossing 
wrinkled-green  peas  with  smooth-yellow  peas.  He  found 
that  smoothness,  S,  is  dominant  over  wrmkledness,  W, 
and  that  yellow  coloi^  F,  is  dominant  over  green,  G, 

\ 


108  GENETICS 

or,  as  it  would  be  stated  according  to  the  presence  or 
absence  theory,  smoothness  is  a  positive  character 
which  fills  out  the  seed-coat  to  plumpness  while  its 
absence  leaves  a  wrinkled  coat,  and  yellowness  is  a 
positive  character  due  to  a  fading  of  the  green  which 
causes  the  yellow  to  be  apparent.  In  the  absence  of 
this  green-fading  factor,  or  determiner,  the  green  of 
course  appears. 

If  smooth-yellow,  SY,  and  wrinkled-green,  WG,  are 
crossed,  all  the  offspring  are  smooth-yellow,  but 
they  carry  concealed  the  recessive  determiners  for 
wrinkledness  and  greenness  according  to  the  formula 
S(W)lf(G).  When  the  determiners  of  these  cross- 
breds  segregate  out  during  the  maturation  of  the  germ- 
cells,  they  may  recombine  so  as  to  form  four  possible 
double  gametes,  namely,  smooth-yellow,  5F,  and 
wrinkled-green,  WG,  which  are  exactly  like  the  grand- 
parental  determiners  from  w_hich  they  arose,  and  in 
addition,  two  entirely  new  combinations,  smooth-green, 
SG,  and  wrinkled-yellow,  WY. 

Since  the  male  and  the  female  cross-breds  are  each 
furnished  with  these  four  possible  gametic  combina- 
tions, the  possible  number  of  zygotes  formed  by  their 
union  will  be  sixteen  (4X4=16).  That  is,  the  mono- 
hybrid  proportion  of  3  to  1  in  dihybrid  combinations 
is  squared,  (3+l)2=16. 

It  of  course  does  not  follow  that  the  offspring  in 
dihybrid  crosses  will  always  be  sixteen  in  number,  or 
that  they  will  always  conform  strictly  to  the  theoreti- 
cal expectation  of  (3+1  )2.  The  offspring  obtained 
undoubtedly  obey  the  laws  of  chance,  but  the  greater 


MENDELISM  109 

the  number  of  offspring,  the  nearer  they  come  to  fall- 
ing into  the  expected  grouping. 

The  sixteen  possible  zygotes  resulting  from  a  dihybrid 
cross  will  give  rise  to  sixteen  possible  kinds  of  indi- 
viduals which  in  turn,  as  will  be  demonstrated  directly, 
present  four  kinds  of  phenotypic  and  nine  kinds  _o£ 
genotypic  constitutions. 

A  dihybrid  mating,  using  the  same  symbols  em- 
ployed in  the  case  just  described,  would  be  expressed 
algebraically  as  follows:-*** 

SG+        WY+        /SF+        WG  =  all  the  possible  egg  gametes 
SG+        WY+        SY+        WG  =  all  the  possible  sperm  gametes      • 
80SG+  80WY+  SGSY+  SGWG 

SGWY  +  WYWY  +   WYSY+   WYWG 

SGSY  -f   WYSY  +SYSY+  SYWG 

SO  WO +   WYWG +  SYWG+WGWQ 

SGSG+2SGWY+28G8Y+2SGWG+WYWY+2WYSY-\-2WYWG+SYSY+28YWG-}-WGWG 
\+~*  V~ 

The  second  and  the  ninth  items  in  this  result  are 
alike;  by  combining  them  the  revised  result  reads: — 

>gOSG + 4SGWf+  2SGSY+ 2SGWG+WTWY  ^T 

There  are  then  these  nine  different  combinations 
of  germinal  characters  or  nine  different  genotypes 
in  any  dihybrid  cross.  By  placing  the  recessive  char- 
acters in  parentheses  whenever  the  corresponding 
dominant  is  present,  to  indicate  that  the  dominant 
causes  the  former  to  recede  from  view,  these  nine  geno- 
types may  be  combined  into  four  phenotypes  as  shown 
in  the  table  at  the  top  of  page  110. 

From  this  analysis  it  may  be  said  that  the  Mendelian 
ratio  for  a  typical  dihybrid  is  phenotypically 
that  for  a  moriohybrid,  as  we  have 


110 


GENETICS 


Phenotypes 


Genotypes 


9SY 


SYSY 


3  SO 


SGSG 


3WY 


WYWY 


already  seen,  is  phenotypically  3:1.     This  expected 
ratio  corresponds   essentially  with  the  actual   results 


SG        WY         SY        WG 

*              *              \              I 

SG 

WY   , 

SY  . 

WG 

& 

SG 

SG 

i         /7\ 

SG 

o,  •* 
o1  ar 

WY 

© 

WY 
V¥— 

© 

SY 

WG, 

'    ® 

—  WY 

sp 

WY 

SY 

WG 

SY 

.\SY 

SY 

4  SY 

SG 

lyY  << 

SY  ' 

WG 

\WG 

.  WG 

WG 

WG 

© 

® 

® 

® 

FIG.  18. — Diagram  to  illustrate  the  possible  combinations  arising 
in  the  second  filial  generation  (F2)  following  a  cross  between 
yellow-smooth,  YS,  and  green-wrinkled,  G^V,  peas. 

which  Mendel  obtained  in  crossing  smooth-yellow  and 
wrinkled-green  peas. 

Figure  18  presents  a  graphic  representation  of  the 
different  combinations  resulting  from  a  dihybrid  cross 
following  the  checkerboard  plan  used  in  Figure  16 
to  illustrate  monohybrids. 

The    nine    genotypes    and    four    phenotypes    which 


MENDELISM 


111 


result  from  a  dihybrid  cross  are  shown  in  the  following 
table: 


Number  in 
Each  Class 

GENOTYPE 

Number  of 
Squares  in 
Fig.  18 

PHENOTYPE 

Number  in 
Each  Class 

1 

8Y8Y 

11 

8Y 

9 

2 

(W)YSY 

7:10 

2 

S(G)SY 

3  -9 

4 

8(G)(W)Y 

2  •  &  •  12  •  15 

1 

SGSG 

1 

8G 

3 

2 

SG(W)G 

13  -4 

I 

WYWY 

6 

WY 

3 

2 

WYW(G) 

8-14 

1 

WGWG 

16 

WG 

1 

16 

16 

Another  illustration  of  dihybridism  is  shown  in  Fig- 
ures 19  and  20  based  upon  data  furnished  by  the 
Davenports.1  In  the  matings  given  here,  dark  or 
pigmented  hair,  represented  by  the  solid  black  circles, 
is  dominant  over  light-colored,  that  is,  unpigmented  or 
slightly  pigmented  hair,  symbolized  by  the  open  circles, 
while  curly  hair  is  dominant  over  straight,  represented 
by  crooked  and  straight  lines  respectively  in  the  dia- 
gram. In  other  words,  the  presence  of  pigment  is 
dominant  over  the  absence  of  pigment,  while  the  factor 
that  causes  curliness  is  dominant  over  the  absence  of 
this  factor,  with  respect  to  human  hair. 

1  "Heredity  of  Eye-color  in  Man,"  Science,  N.  S.  26,  p.  589, 
1907;  "Heredity  of  Hair  Form  in  Man,"  Amer.  Nat.  42,  p.  341, 
1908.  Davenport,  C.  B.  and  G.  C. 


GENETICS 


When    a    homozygous    individual    with    dark    curly 
hair  crosses  with  a  homozygous  individual  with  light 


\\r\  f7  /  /  s  ... 
x V  //--/  /-"  /  .-••• 

X  \  v  AA'  /  .'7     s 


O  =  Light 
B  <=  Curly 
—  =  Straight 


FIG.  19. — The  heredity  of  human  hair  according  to  data  by  C.  B. 
and  G.  C.  Davenport.  The  arcs  represent  the  somatoplasms 
of  four  individuals.  Within  the  arcs  are  the  gametes  formed 
by  these  individuals.  The  dominant  character  is  placed  on 
the  outside  of  the  arc  where  it  will  be  visible. 

straight  hair,  all  the  offspring  have  dark  curly  hair. 

The   dark   curly-haired   individuals    of   this    second 

generation,  however,  are  heterozygous  with  respect  to 


MENDELISM 


113 


each  of  these  two  hair  characters.  When  any  two 
individuals  having  this  particular  genotypic  composi- 
tion mate,  therefore,  they  may  produce  any  one  of  four 
possible  phenotypes — dark  curly,  dark  straight,  light 
curly  or  light  straight 
haired  individuals. 
These  four  phenotypes 
in  turn  will  present  nine 
different  genotypic 
combinations  out  of 
sixteen  possible  cases, 
as  shown  in  Figure  20. 

Figure  19  further- 
more serves  to  make 
clear,  first,  the  distinc- 
tion between  somato- 
jplasm  and  germplasm ; 
second,  the  maturation 
of  germ-cells ;  third, 
the  segregation  of 
gametes ;  and  fourth, 
jthe  formation  of  zy- 
gotes  in  sexual  repro- 
duction. 

The  cells  of  the  so- 


in  each 
class 

Genotype 

Phenotype 

in  each 
class 

4 

(*^\ 

Dark  curly 

9 

2 

© 

2 

/^§\ 

1 

© 

1 

© 

Dark  straight 

3 

2 

© 

1 

© 

Light  curly 

3 

2 

(^\ 

1 

© 

Light  straight 

1 

16 

16 

FIG.  20. — Diagrams  showing  the 
possible  genotypic  and  pheno- 
typic  combinations  resulting 
when  two  heterozygous  individu- 
als with  dark  curly  hair  mate. 
Symbols  are  the  same  as  in 
Figure  19. 


matoplasm  are  represented  as  making  up  the  arcs  within 
which  are  inclosed  the  germ-cells  after  their  reduction 
through  maturation,  which  results  in  giving  to  each 
germ-cell  half  the  number  of  determiners  that  are 
present  in  the  somatic  cells. 

It  will  be  remembered  that  when  two  gametes,  or 


114  GENETICS 

mature  germ-cells,  unite,  they  form  a  zygote  having  the 
proper  number  of  determiners  normal  to  the  species 
in  question  instead  of  double  that  number.  Symbols  for 
dominant  characters  in  the  diagram  are  placed  on  the 
outside  of  the  somatic  arcs,  because  these  are  the  char- 
acters that  are  visible  or  phenotypic,  while  the  non- 
apparent  recessives  are  placed  on  the  inside  out  of 
sight. 

11.  THE  CASE  OF  THE  TRIHYBRID 

Mendel  went  even  further  and  computed  the  possi- 
bilities which  would  result  when  two  parents  were 
crossed  differing  from  each  other  with  respect  to  three 
unit  characters.  He  found  that  the  results  actually 
obtained  by  breeding  closely  approximated  the  theo- 
retical expectation. 

This  expectation  in  the  case  of  a  trihybrid  cross  is 
that  the  cross-breds  resulting  will  all  exhibit  the  three 
dominant  characters,  while  their  genotypic  constitution 
will  include  six  factors,  namely,  these  three  dominant 
characters  plus  their  corresponding  recessives  or  "ab- 
sences." 

Cross-breds  of  the  first  generation  will,  therefore, 
have  eight  possible  kinds  of  triple  gametes  and  when 
interbred  may  form  a  possible  range  of  sixty-four 
(8X8)  different  zygotes,  which  corresponds  to  a 
monohybrid  raised  to  the  third  power  (3-f-l)3.  These 
sixty-four  zygotes  group  together  in  eight  different 
phenotypes  and  twenty-seven  different  genotypes  as 
shown  on  page  116. 

The  trihybrid  cross  with  its  resulting  combinations 


MENDELISM 


115 


is  well  illustrated  by  Castle's  work  on  guinea-pigs 
which  confirms  the  Mendelian  hypothesis  on  an  extensive 
scale.  In  Figure  21  dominant  characters  are  repre- 


?t 


rSP-*- 


rsp- 


RSP    RsP  RSp    Rsp    rSP    rsP    rSp     rap 
»         I         *         1         1         1         t         1 

RSP 
RSP 

RsP 
RSP 

RSp 
RSP 

Rsp 
RSP 

rSP 
RSP 

rsP 
RSP 

rSp 
RSP 

rsp 
RSP 

RSP 
RsP 

RsP 
RsP 

RSp 
RsP 

Rsp 
RsP 

rSP 
RsP 

rsP 
RsP 

rSp 
RsP 

rsp 
RsP 

RSP 
RSp 

RsP 
RSp 

RSp 
RSp 

Rsp 
RSp 

rSP 
RSp 

rsP 
RSp 

rSp 
RSp 

rsp 
RSp 

RSP 
Rsp 

RsP 
Rsp 

RSp 
Rsp 

Rsp 
Rsp 

rSP 
Rsp 

rsP 
Rsp 

rSp 
Rsp 

rsp 
Rsp 

RSP 
rSP 

RsP 
rSP 

RSp 
rSP 

Rsp 
rSP 

rSP 
rSP 

rsP 
rSP 

rSp 
rSP 

rsp 
rSP 

RSP 
rsP 

RsP 
rsP 

RSp 
rsP 

Rsp 
rsP 

rSP 
rsP 

rsP 
rsP 

rSp 
rsP 

rsp 
rsP 

RSP 

rSp 

RsP 

rSp 

RSp 
rSp 

Rsp 

rSp 

rSP 

rSp 

rsP 

rSp 

rSp 
rSp 

rsp 
rSp 

RSP 
rsp 

RsP 
rsp 

RSp 
rsp 

Rsp 
rsp 

rSP 
rsp 

rsP 
rsp 

rSp 
rsp 

rsp 
rsp. 

FIG.  21.— Diagram  showing  the  possible  combinations  in  a  guinea- 
pig  trihybrid  of  the  F2  generation.  R,  resetted  coat;  r,  non- 
rosetted  coat  (absence  of  R)  ;  S,  short  hair;  s,  angora  hair 
(absence  of  S)  ;  P,  pigmented;  p,  albino  (absence  of  pig- 
ment). The  eight  possible  triple  gametes  of  each  parent  are 
placed  in  the  upper  and  left  hand  margins  respectively. 
Each  of  the  sixty-four  squares  represents  a  possible  zygote 
or  fertilized  egg,  having  received  a  triple  gamete  from  each 
parent. 

sented  by  capital  letters,  while  recessives  or  absences 
are  indicated  by  corresponding  small  letters. 

When  a  smooth,  or  non-rosetted   (r),  short-haired 
(S),   pigmented    (P)    guinea-pig    is    crossed    with    a 


116 


GENETICS 


Number  in 
each  class 

GENOTYPE 

PHENOTTPE 

Number  in 
each  class 

1 

SS  PP  RR 

SPR 
Short,  pigmented,  resetted 

27 

2 
2 
4 

SS  Pp  RR 

Ss   PP  RR 

Ss   Pp   RR 

2 

SS  PP  Rr 

4 

SS  Pp  Rr 

4 

8 

Ss  PP  Rr 

Ss  Pp  Rr 

1 

SS  pp   RR 

SpR 
Short,  albino,  resetted 

9 
9 

2 
2 

Ss  pp    RR 

SS  pp    Rr 

4 

Ss   pp    Rr 

1 

ss    PP  RR 

sPR 
Angora,  pigmented,  resetted 

2 

ss    Pp  RR 

2 

ss    PP  Rr 

4 

ss    Pp  Rr 

1 

SS  PP  rr 

SPr 
Short,  pigmented,  non-rosetted 

9 

2 

SS  Pp   rr 

2 
4 

Ss   PP  rr 

Ss   Pp  rr 

1 

ss    pp    RR 

spR 
Angora,  albino,  resetted 

3 

2 

ss    pp    Rr 

1 

SS  pp    rr 

Spr 
Short,  albino,  non-rosetted 

3 

2 

Ss   pp    rr 

1 

ss    PP  rr 

sPr 

Angora,  pigmented,  non-resetted 

3 
1 

2 

ss    Pp  rr 

1 

ss    pp    rr 

spr 
Angora,  albino,  non-rosetted 

64 

64 

MENDELISM  117 

resetted  (#),  long-haired  (s),  albino  (p)  guinea-pig, 
all  the  offspring  appear  to  be  of  one  phenotypic 
constitution,  namely,  resetted,  short-haired,  and  pig- 
mented  (RSP).  Their  genotypic  constitution  is  rep- 
resented by  the  formula  RrSsPp.  These  six  factors 
may  form  eight  possible  triple  gametes,  as  follows : 
RSP,  RsP,  RSp,  Rsp,  rSP,  rSp,  rsP,  rsp.  When  two 
germ-cells  each  made  up  of  these  eight  triple  gametes 
unite  in  sexual  reproduction,  they  will  give  rise  to  sixty- 
four  (8X8)  possible  zygotes  as  displayed  in  Figure  21. 

An  analysis  of  Figure  21  shows  among  the  offspring 
eight  different  phenotypes  in  the  ratio  of  27 :9 :9 :9 :3 : 
3:3:1  and  27  different  genotypes  in  the  proportions 
indicated  on  page  116.  The  order  of  the  three  pairs 
of  symbols  is  changed  from  that  in  Figure  21  to 
emphasize  the  fact  that  with  independent  unit  char- 
acters the  order  is  immaterial. 

Sketches,  drawn  from  photographs  in  Castle's  "Ge- 
netics and  Eugenics,"  of  the  eight  phenotypically  differ- 
ent guinea-pigs  here  described  are  shown  in  Figure  22. 

12.  SUMMARY 

Three  principles  are  concerned  in  Mendel's  law: 
/  independent  unit  characters,  dominance,  and  segrega- 
tion. 

a.  Independent  Unit  Characters.  An  organism, 
although  acting  together  as  a  physiological  and  mor- 
phological whole,  may  be  regarded  from  the  point  of 
view  of  heredity  as  consisting  of  a  large  number  of 
independent  heritable  unit  characters. 


118 


GENETICS 


SPR 


SpR 


sPR 


SPr 


spR 


Spr 


sPr 


spr 


FIG.  22. — The  eight  phenotypically  different  kinds  of  guinea-pigs 
in  the  Fa  generation  of  a  trihybrid.  S  =  short  hair ;  s  =  long 
hair  or  angora ;  P  =  pigmented  coat ;  p  =  non-pigmented  coat 
or  albino ;  R  =  rough  or  resetted  coat ;  r  =  smooth  coat. 
Drawn  from  Castle's  photographs  by  C.  J.  Fish. 


MENDELISM 

b.  Dominance.     In  every  individual  there  are  two 
determiners  for  every  unit  character,  one  derived  from 
each  parent.    If  this  pair  is  different,  i.  <?.,  if  the  zygote 
is  a  heterozygote,  one  dominates  the  other  and  deter- 
mines the  apparent  character  of  the  organism. 

The  alternative  recessive  characters,  although  they 
may  be  present  in  the  germplasm,  are  unable  to  be- 
come manifest  in  the  somatoplasm  so  long  as  the  domi- 
nant characters  are  present.  When,  however,  the  domi- 
nant determiner  is  absent,  and  the  recessive  is  dupli- 
cated, the  recessive  character  becomes  manifest. 

c.  Segregation.    The  determiners  of  unit  characters, 
although  they  may  be  intimately  associated  together  in 
the  individual,  during  the  complicated  process  of  ma- 
turation that  always  precedes  the  formation  of  a  new 
individual,  separate  or  segregate  out  as  if  independent 
of  each  other  and  thus  are  enabled  to  unite  into  new 
combinations. 

13.  THE  PRACTICAL  APPLICATION 

Although  the  ratios  for  more  than  a  trihybrid  were 
computed  by  Mendel,  the  experimental  test  was  not 
carried  out  by  him,  since  it  involves  such  large  and  com- 
plicated proportions. 

In  the  case  of  four  differing  unit  characters  in  the 
parental  generation,  the  offspring  of  the  quadruple 
hybrids  derived  from  such  an  ancestry  would  include 
256  or  (3+1)4  possibilities  instead  of  64  or  (3+1)3, 
as  in  the  case  of  trihybrids.  When  ten  differing  char- 
acters are  combined  in  the  parental  generation, 


120  GENETICS 

there  would  result  over  a  million  possible  kinds  of 
offspring  among  the  hybrids  of  the  second  filial 
generation,  (3+l)10=l,048,576. 

From  the  foregoing  it  is  apparent  that  in  practical 
breeding  the  only  hope  lies  in  dealing  with  not  more 
than  one  or  two  characters  at  a  time.  Since  unit 
characters  usually  behave  independently  of  each  other, 
one  may  breed  for  a  single  character  until  it  is  segre- 
gated out  in  a  homozygous,  that  is  pure,  condition, 
and  then  in  the  same  way  obtain  a  second  character,  a 
third,  and  so  on. 

This  is  not  difficult  if  the  character  sought  is  a 
recessive  for,  in  that  case  it  is  already  homozygous  or 
pure  and  consequently  appears.  When  a  character  is 
dominant  it  takes  longer  to  determine  whether  the 
individual  is  heterozygous  (hybrid)  or  homozygous 
(pure). 

14.  CONCLUSION 

The  Mendelian  method  is  an  attempt  to  analyze  the 
behavior  of  a  particular  characteristic  in  heredity 
rather  than  to  get  at  the  lump  performance  of  the  in- 
dividual as  a  whole.  Herein  lies  the  scientific  control  of 
heredity  which  the  trinity  of  Mendelian  principles 
namely,  independent  unit  characters,  segregation,  and 
dominance,  has  placed  in  human  hands.  Following  this 
method  there  can  be  obtained  in  a  few  generations  of 
properly  directed  crosses,  combinations  of  characters 
united  in  one  strain  that  formerly  were  never  obtained 
at  all  or  were  only  hit  upon  by  the  merest  chance  at 
long  intervals. 


CHAPTER  VI 

THE  PURE  LINE  AND  SELECTION 
1.  GALTON'S  LAW  OF  REGRESSION 

GALTON  was  one  of  the  first  *  to  attempt  to  express 
mathematically  the  relationship  between  parents  and 
offspring  by  means_ojF__ir^atmg_^tatisjically  a  single 
unit  character.  According  to  Galton,  a  mathematical 
expression  of  the  relationship  between  two  generations 
should  serve  as  a  corner-stone  of  heredity. 

What  Galton  did  was  to  take  human  stature  as  a 
unit  character  in  comparing  204  English  parents  and 
their  928  adult  offspring,  because  human  stature  is 
not  complicated  by  environmental  influences  and  is 
consequently,  a  purely  hereditary  matter. 

The  results  of  his  measurements  expressed  in  inches 
are  shown  in  Figure  23  in  which  the  circles  connected  by 
the  diagonal  line  represent  the  graded  parental  heights, 
while  the  arrowpoints  indicate  the  average  heights  of 
the  offspring  in  each  group. 

This  illustrates  Galton's  Law  of  Regression  or  the 
tendency  in  successive  generations  toward  mediocrity. 
The  law  may  be  stated  as  follows : — 

Average  parents  tend  to  produce  average  children; 
minus  parents  tend  to  produce  minus  children;  plus 

1  "Hereditary  Genius,"  1869. 

121 


122  GENETICS 

parents  tend  to  produce  plus  children ;  but  the  progeny 
of  extreme  parents,  whether  plus  or  minus,  inherit  the 
parental  peculiarities  in  a  less  marked  degree  than  the 
latter  were  manifested  in  the  parents  themselves. 


FIG.  23. — Scheme  to  illustrate  Gallon's  law  of  regression.  The 
circles  represent  graded  groups  of  parental  height  while  the 
arrowpoints  indicate  the  average  heights  attained  by  the  re- 
spective offspring.  The  offspring  of  undersized  parents  are 
taller,  and  of  oversized  parents  are  shorter  than  their  respec- 
tive parents.  Based  on  data  from  Galton. 


2.  THE  IDEA  OF  THE  PURE  LINE 

It  was  Galton's  law  of  regression  that  suggested 
to  the  Danish  botanist  Johannsen  a  possible  means  of 
controlling  heredity.  In  his  mind  arose  the  question 
whether  it  would  not  be  possible  by  continually  breeding 


THE  PURE  LINE  AND  SELECTION 

from  plus  parents,  granting  that  plus  parents  produce 
plus  offspring  and  making  allowance  for  some  regres- 
sion to  type,  to  shove  over  the  offspring  more  and  more 
into  the  plus  territory  and  so  to  establish  a  plus  race. 

To  test  this  hypothesis,  Johannsen  selected  beans, 
Phaseolus,  with  which  to  experiment,  since  this  group 
of  plants  is  self-fertilizing,  prolific,  and  easily  measur- 
able. Somewhat  to  his  surprise,  the  beans  refused  to 
shove  over  as  much  as  expected.  That  is,  big  beans 
did  not  yield  principally  big  offspring,  nor  little  beans 
little  offspring,  according  to  the  expectation,  although 
they  each  produced  offspring  that  varied  in  the  manner 
of  fluctuating  variability  around  an  average  unlike  the 
parental  type.  This  gave  Johannsen  the  idea  that  he 
was  using  mixed  material,  so  he  next  isolated  the  prog- 
eny of  single  beans,  which,  being  self -fertilized  for  many 
generations,  each  constituted  unmistakably  a  single 
hereditary  line.  In  this  way  nineteen  beans,  now  fa- 
mous, became  the  known  ancestors  of  Johannsen's 
original  nineteen  "pure  lines,"  a  further  study  of  which 
has  led  the  way  to  some  of  the  most  brilliant  biological 
discoveries  of  recent  years. 

A  pure  line  has  been  defined  by  Johannsen  as  "the 
descendants  from  a  single  homozygous  organism  ex- 
clusively propagating  by  self-fertilization,"  and  more 
briefly  by  Jennings  as  "all  the  progeny  of  a  single 
self-fertilized  individual." 

It  should  be  pointed  out,  however,  that  this  technical 
idea  of  a  "pure  line"  is  not  at  all  the  same  as  that  which 
the  breeder  has  in  mind  when  he  uses  the  same  term. 
The  nearer  individuals  can  be  bred  to  conform  to  an 


GENETICS 

arbitrary  standard  agreed  upon,  the  better  they  illus- 
trate the  stock-breeder's  idea  of  a  pure  line.  For  ex- 
ample, in  "The  Standard  of  Perfection,"  a  book  pub- 
lished by  the  American  Poultry  Association,  there  are 
recognized  42  breeds  and  121  varieties  of  chickens. 
To  belong  to  any  particular  breed  in  this  gallinaceous 
Blue  Book  the  chicken  must  look  the  part  regardless  of 
its  germinal  derivation. 

To  the  biologist,  on  the  contrary,  the  pure  line  is 
like  an  imaginary  mathematical  concept  depending  en- 
tirely upon  similarity  of  the  determining  hereditary 
complex.  The  biologist's  pure  line  is  genotypic.  The 
stock-breeder's  is  phenotypic,  a  difference  of  definition 
which  has  given  rise  to  considerable  confusion. 

In  a  certain  general  way  it  will  be  seen  that  the  pure 
line  stands  over  against  mutation,  since  it  is  concerned 
with  the  conservative  maintenance  of  type  while  muta- 
tion attempts  to  change  it. 

The  inevitable  monotony  of  a  pure  line  may  be  con- 
siderably masked  by  individual  somatic  modification. 
DeVries  has  said  paradoxically,  "The  pure  line  is  com- 
pletely constant  and  extremely  variable."  That  is,  it 
is  "completely  constant"  except  for  mutations,  and 
it  is  "extremely  variable"  in  the  somatic  development 
that  may  be  attained  by  separate  individuals. 

3.  JOHANNSEN'S  NINETEEN  BEANS 

To  return  to  experiments  with  beans,  Johannsen 
found  out  that  the  progeny  of  every  one  of  his  pure 
lines  varied  around  its  own  mean,  which  was  different 


THE  PURE  LINE  AND  SELECTION     185 

in  each  of  the  nineteen  instances.  When,  however,  ex- 
tremes from  any  pure  line  series  were  selected  and  bred 
from,  the  results  showed  complete  regression  away  from 
the  extreme  condition  of  the  parent  bean  back  to  the 

Average  of_ 
all  progeny" 
Weight  of  parent  seed  \ 


10203040606070    10  20  80  40  BO  60  70   10203040506070    10203040606070    10208040606070 

Pure  line  number >U  VU  XV  XVIII 

FIG.  24. — The  result  of  selection  in  four  pure  lines  of  beans.  The 
vertical  columns,  representing  the  average  progeny  from 
different  sized  parents  all  derived  from  the  same  pure  line, 
contain  groups  nearer  alike  than  the  horizontal  columns,  rep- 
resenting progeny  from  the  same  sized  parents,  but  different 
pure  lines.  All  the  numbers  indicate  centigrams.  Data  from 
Johannsen. 


type  of  the  entire  pure  line  in  question.  That  is, 
selection  within  a  pure  line  is  absolutely  without  effect 
in  modifying  a  particular  character  in  the  offspring  of 
the  line  in  question. 

This  is  illustrated  in  Figure  24  in  which  the  results 
of  selecting  for  size  in  the  year  1902  is  shown  for  four 


126 


GENETICS 


pure  lines  only.  The  average  for  each  pure  line  is 
given  at  the  top  of  its  column.  When,  for  example, 
beans  weighing  60  eg.  were  selected  from  pure  lines  II, 
VII,  and  XV,  the  average  weights  of  their  progeny 
were  56.5,  48.2,  and  45.0  eg.  respectively,  which  in 
each  instance  is  nearer  to  the  average  for  the  pure 
line  than  to  the  weight  of  the  parental  seed. 

It  will  be  seen  at  once  that  the  averages  in  the  vertical 
columns  are  nearer  alike  than  the  averages  in  the  hori- 
zontal columns.  In  other  words,  the  beans  bred  true 
to  their  pure  line  rather  than  to  their  fluctuating 
parent. 

As  a  further  example  of  this  law,  take  the  result 
of  selection  for  six  years  in  pure  line  I  as  shown  in 
the  accompanying  table  and  in  Figure  25. 


HARVEST  YEAR 

MEAN  WEIGHT  OF 
SELECTED  PARENT  SEED 

MEAN  WEIGHT  or 
OFFSPRING 

Minus 

Plus 

From  Minus 
Parent 

From  Plus 
Parent 

1902 
1903 
1904 
1905 
1906 
1907 

60 
55 
50 
43 
46 
56 

70 
80 
87 
73 

84 
81 

63.15 
75.19 
54.59 
63.55 
74.38 
69.07 

64.85 
70.88 
56.68 
63.64 
73.00 
67.66 

It  is  evident,  for  instance,  that  in  1907  the  smallest 
beans,  weighing  an  average  of  56  eg.,  gave  an  average 
progeny  weighing  69.07  eg.,  while  the  largest  ones 
for  the  same  year,  weighing  an  average  of  81  eg.,  pro- 
duced nearly  the  same  average  in  their  progeny  as  did 
the  smallest  beans,  that  is,  67.66  eg. 


THE  PURE  LINE  AND  SELECTION    127 

Incidentally  all  the  progeny  from  both  large  and 
small  parents  averaged  notably  less  in  1904  than  all 
the  progeny  from  large  and  small  parents  in  1906, 
a  result  due  to  a  "poor  year"  when  certain  factors 
of  environment  were  unfavorable.  Such  unfavorable 
conditions,  however,  are  known  to  influence  in  no  way 
the  hereditary  qualities  of  the  beans.  Thus  it  appears 
that,  although  the  progeny  of  a  pure  line  present 
plenty  of  variations  of  the  fluctuating  type,  due  prob- 
ably to  environmental  differences  in  nutrition,  moisture, 
etc.,  such  variations  are  quite  ineffectual  so  far  as 
inheritance  is  concerned,  and  it  makes  no  difference 
whether  the  largest  or  the  smallest  beans  within  a  pure 
line  are  selected  from  which  to  breed,  the  result  will  be 
the  same,  in  that  there  is  a  complete  return  to  medioc- 
rity or  type  with  no  "inheritance"  of  the  parental  modi- 
fication. As  a  matter  of  fact  in  1903,  1906  and  1907 
the  lighter  parents  gave  heavier  progeny  than  the 
heavier  parents. 

It  will  be  seen  at  once  that  here  is  a  discovery  of 
far-reaching  importance  which  may  require  us  to 
reconstruct  certain  cherished  ideas  about  the  part 
played  in  the  evolution  of  species,  as  well  as  in  heredity, 
by  natural  selection. 

4.  THE  DISTINCTION  BETWEEN  A  POPULATION  AND 
A  PURE  LINE 

A  mixture  of  pure  lines  has  been  called  a  population 
(Johannsen). 

It  is  not  possible  to  distinguish  by  inspection  a  group 


128 


GENETICS 


THE  PURE  LINE  AND  SELECTION    129 

of  individuals  composing  a  pure  line  from  a  group 
making  up  a  population,  since  both  may  be  phenotypi- 
cally  alike.  Fluctuations  about  the  average  occur  in 
both  cases  with  no  appreciable  difference  in  character, 
although  such  fluctuations,  when  they  occur  within  a 
pure  line,  are  simply  somatic  differences  caused  in 
general  probably  by  modifications  in  nutrition  or  some 
other  external  factor  of  environment,  while  fluctua- 
tions in  a  population  include  not  only  modifications  of 
this  transient  nature,  but  also  permanent  hereditary 
differences  due  to  germinal  differences  in  the  various 
pure  lines  of  which  the  population  is  composed. 

Johannsen  has  made  the  distinction  between  pure 
lines  and  populations  clear  by  the  following  figure 
(Fig.  26),  in  which  five  pure  lines  of  beans  are  com- 
bined artificially  to  form  a  population. 

The  beans  which  make  up  the  pure  lines  noted  in 
this  figure  are  represented  inclosed  within  inverted 
test  tubes.  The  beans  in  any  single  tube  are  all  of 
one  size.  Tubes  vertically  superimposed  upon  each 
other  also  contain  only  beans  of  one  size. 

Thus  it  is  seen  that  what  may  be  a  rare  size  of 
bean  in  one  line,  for  instance  that  in  the  left-hand 
tube  of  pure  line  3,  may  be  identical  with  the  com- 
monest size  in  another  line,  as  pure  line  2.  The  five 
pure  lines  represented  in  Figure  26  are  combined  in 
a  population  at  the  bottom  of  the  figure.  In  this 
population  array  the  five  pure  lines  are  hidden. 

Hence,  while  selection  within  a  pure  line  has  no  he- 
reditary influence,  it  is  evident  that  selection  within  a 
population  may  shift  or  move  over  the  type  of  the 


130 


GENETICS 


progeny  obtained,  in  the  direction  of  the  selection  simply 
by  isolating  out  a  pure  line  of  one  type.  Thus  beans 
Pure  Line  chosen  from  the 

extreme  1  e  f  t- 
hand  test  tube  in 
the  population 
cited  would  be- 
long only  to  pure 
line  2,  while  those 
taken  from  the 
extreme  right- 
hand  test  tube 
could  belong  only 
to  pure  line  3. 

Galton's  "law 
of  regression," 
namely,  that 
minus  parents 
give  minus  off- 
spring and  plus 
parents  plus  off- 
spring, with  a 
tendency  to  re- 
version from  gen- 
eration to  gener- 
ation, depends 

FIG.  26.— Diagrams  showing  five  pure  lines  simply     upon     a 
and  a  population  formed  by  their  union.  r  tj        r 

The  beans  of  each  pure  line  are  repre-  partial    but    not 
sented  as  assorted  into  inverted  test  tubes  ••   •.  i 

making  a  curve  of  fluctuating  variability.  < 

Test  tubes  containing  beans  of  the  same  tion  of  pure  lines 
weight    are    placed   in   the   same    vertical  f  , 

row.     After  Johannsen.  out  of  a  popula- 
tion. 


THE  PURE  LINE  AND  SELECTION     181 

From  this  distinction  between  pure  lines  and  popula- 
tions it  is  clear  why  breeders  in  selecting  for  a  particu- 
lar character  out  of  their  stock  need  to  keep  on  select- 
ing continually  in  order  to  maintain  a  certain  standard. 
As  soon  as  they  cease  this  vigilance,  there  is  a  "reversion 
to  type"  or,  as  they  say,  "the  strain  runs  out,"  which 
means  that  the  pure  lines  become  lost  in  the  mixed  popu- 
lation which  inevitably  results  as  soon  as  selective  iso- 
lation of  the  pure  line  ceases. 

Such  reversion  must  always  be  the  case  in  dealing 
with  a  population  made  up  of  a  mixture  of  pure  lines, 
for  only  by  the  isolation  of  pure  lines  can  the  constancy 
of  a  character  be  maintained.  When,  however,  a  pure 
line  is  once  isolated,  then  all  the  members  of  it,  large 
as  well  as  small,  are  equally  efficient  in  maintaining  the 
pure  line  in  question,  regardless  of  their  phenotypical 
constitutions. 

Conceding  that  natural  history  and  common  usage  as 
well  as  the  older  theories  of  heredity  are  concerned  with 
phenotypic  constitution  of  organisms,  we  are  now 
coming  to  see  more  clearly  than  before  that  heredity 
must  always  be  a  case  of  similarity  in  origin,  that  is, 
in  germinal  composition,  and  that  similarity  in  appear- 
ance by  no  means  always  indicates  similarity  in  origin 
or  true  relationship. 

The  assumption  that  similarity  in  appearance  does 
indicate  relationship  has  been  made  the  foundation  of 
many  conclusions  in  comparative  anatomy  and  phy- 
logeny,  but  to  the  modern  student  of  genetics  who  places 
his  faith  in  things  as  they  are,  rather  than  in  things  as 
they  seem  to  be,  conclusions  based  upon  phenotypical 


GENETICS 

distinctions  alone  have  in  them  a  large  source  of  error 
which  must  be  taken  into  account. 

In  a  museum  of  heredity,  should  such  a  collection 
ever  be  assembled,  the  specimens  would  not  be  arranged 
phenotypically  as  they  are  in  an  ordinary  museum 
where  things  that  look  alike  are  placed  together  as  if 
in  bonds  of  relationship,  but  they  would  be  arranged 
historically  from  a  genetic  point  of  view  to  show  their 
true  origin  one  from  another. 

5.  CASES  SIMILAR  TO  JOHANNSEN'S  PURE  LINES 

Although,  according  to  Johannsen,  pure  lines  are 
"the  progeny  of  a  single  self-fertilized  individual,"  it 
is  plain  that  in  at  least  three  other  possible  cases  some- 
thing quite  similar  to  "pure  lines"  may  be  obtained. 

These  are  clones,  partheno  genetic  progeny  and  homo- 
zygous  crosses.  "In  principle  pure  lines,  partheno- 
genetic  reproduction  and  vegetative  propagation  are 
concerned  with  nearly  the  same  situation"  (Morgan). 

First,  in  asexual  reproduction  where  the  progeny 
are  simply  the  result  of  continued  fission  of  the  original 
individual,  a  pure  line  may  be  said  to  continue  from 
generation  to  generation  because  it  is  a  germinally  un- 
changed sequence  of  individuals.  Such  an  asexual 
progeny  is  termed  a  clone  (Webber).  Shull's  definition 
of  a  clone  is  "a  group  of  individuals  of  like  genotypic 
constitution,  traceable  through  asexual  reproductions 
to  a  single  ancestral  zygote,  or  else  perpetually 
asexual." 

Second,   in   cases   of  parthenogenesis,   the  progeny 


THE  PURE  LINE  AND  SELECTION     133 

arising  from  a  single  female  individual  without  the 
customary  maturation  of  the  germ-cells  which  accom- 
panies sexual  reproduction,  constitute  a  pure  line  or  an 
unmixed  strain  because  as  in  clones  there  has  been  no 
segregation  nor  addition  of  outside  germplasm. 

Third,  in  homozygous  crosses  when  two  organisms 
identical  in  their  germinal  determiners  inbreed,  their 
progeny  will  form  a  pure  line  just  as  truly  as  two 
parents  that  are  united  in  a  single  hermaphroditic  in- 
dividual produce  a  pure  line  progeny  as  the  result  of 
self-fertilization. 

In  the  case  of  clones  and  parthenogenesis  it  should  be 
pointed  out  that  the  "pure  line"  is  assured  only  so 
long  as  asexual  reproduction  continues.  It  is  quite 
possible  for  an  organism,  even  heterozygotic  in  composi- 
tion, to  continue  to  breed  true  or  to  produce  an  ap- 
parently pure  line  so  long  as  asexual  methods  are  em- 
ployed. As  soon  as  such  an  organism,  however,  changes 
to  the  sexual  method  of  reproduction,  segregation  of 
characters  may  occur  and  different  combinations  result. 
A  pure  line,  therefore,  implies  freedom  from  admixture 
of  different  germplasm  rather  than  any  necessary 
equality  or  likeness  of  individuals. 

The  different  kinds  of  "pure  lines"  are  diagram- 
matically  represented  in  Figure  27. 

6.  SELECTION  WITHIN  A  PURE  LINE 

The  basic  idea  of  the  pure  line  concept  is  that  every 
member  of  any  pure  line  is  genetically  identical  with 
every  other  member  of  the  same  fraternity,  therefore, 


134 


GENETICS 


THE  PURE  LINE  AND  SELECTION      155 

any  differences  found  between  individuals  of  a  pure  line 
are  entirely  somatic  and  not  hereditary. 

The  importance  of  the  problem  of  pure  line  selection 
for  any  general  consideration  of  the  mechanism  of  evo- 
lution is  at  once  apparent.  There  have  been  many  re- 
cent investigations  besides  those  of  Johannsen  to  test 
the  result  of  selection  within  the  four  kinds  of  "pure 
lines."  Some  of  these  investigations  are  enumerated  in 
the  table  on  pages  136  and  137. 


It  is  apparent  in  the  first  section  of  the  following 
table  that  the  pure  line  sensu  stricto,  that  is,  the  pure 
line  of  Johannsen,  must  be  studied  with  plants  alone, 
since  among  animals  only  certain  highly  specialized 
parasitic  worms,  which  do  not  lend  themselves  readily  to 
selection  experiments,  produce  offspring  by  means  of 
self-fertilization.  The  work  of  the  other  authors 
upon  plants,  mentioned  in  the  first  group  of  the  table,  is 
in  entire  agreement  with  the  work  of  Johannsen. 

The  noteworthy  contribution  of  L.  de  Vilmorin  con- 
sists in  a  detailed  comparison  of  preserved  specimens 
of  certain  pure  lines  of  wheat  which  were  isolated  in 
France  about  1840,  with  their  lineal  descendants  of 
to-day.  In  spite  of  continuous  selection  for  better- 
ment within  these  self-fertilized  strains  during  more  than 
60  years,  their  constancy  has  been  maintained. 

B.  CLONES 

With  respect  to  selection  within  a  clone  there  is  an 
apparent  conflict  of  results. 


136  GENETICS 

THE  RESULTS  OF  SELECTION  WITHIN  A  PURE  LINE 


Kind  of 
pure  line 

Author 

Organism 

Character 
selected 

Result 

Johannsen,  '03 

Beans 

Size 

No  effect 

K 

Barley 

Mealiness  of 

kernel 

«      « 

Progeny 

Nilsson 

Wheat 

Various 

of  a 

Oats 

characters 

«         c< 

single 

Barley 

self- 

Surface  and 

fertil- 

Pearl 

Oats 

Yield  per  acre 

«        « 

ized 

Fruwirth,  '17 

Lentils 

indi- 

Peas 

it         « 

vidual 

Soy  beans 

Lupines 

L.  de  Vilmorin 

Wheat 

«         « 

Wolf,  '09 

Bacteria 

Pigment  pro- 

duction 

«         « 

Barber,  '07 

« 

«         « 

Wiuslow  and 

Walker,  '09 

« 

it             it 

Meader,  '19 

« 

Form,    fer- 

mentative 

reaction, 

virulence 

«        « 

East,  '09-'10 

Potato 

11        it 

Vogler,'14 

Garlic 

((        tl 

Stout,  '15 

Coleus 

Color  pattern 

Effective 

Mendiola,  '19 

Lemna 

Size  and  shape 

of   frond, 

speed  of 

budding 

No  effect 

Jennings,  '08 
"        '16 

Paramecium 
Difflugia 

Size 
Six  shell 

{(             U 

Clones 

characters 

Effective 

Calkins  and 

Gregory,  '13 

Paramecium 

Size,  rate   of 

fission,  etc. 

{< 

Jollos,  '13 

« 

Resistance  to 

arsenical 

poisoning 

No  effect 

Stocking,  '15 

« 

Abnormalities 

Effective  in 

some  lines 

Middleton,  '15 

Stylonychia 

Fission-rate 

Diverse  strain 

from  one 

THE  PURE  LINE  AND  SELECTION     137 

THE  RESULTS  OF  SELECTION  WITHIN  A  PURE  LINE — Continued 


Kind  of 
Pure  line 

Author 

Organism 

Character 
selected 

Result 

Ackert,  '16 

Paramecium 

Size 

No  effect 

Root,  '18 

Centropyxis 

Shell  charac- 

ters, fission- 

rate 

Effective 

Hegner,  '19 

Arcella 

Shell  charac- 

Diverse strains 

ters 

from  one 

Hanel,  '08 

Hydra 

No.of  tentacles 

No  effect 

Lashley,  '16 

w 

U       It                   " 

«      «< 

Woltereck,  '09 

Hyalodaph- 

Length  and 

Temporary 

nia 

shape  of 

temperature 

"head" 

effect 

Agar,  '13 

Simoceph- 

No  effect 

alus 

"      '14 

Aphids 

t<      « 

Partheno- 
genetic 

Ewing,  '14 

« 

Length  of 
honeydew 

prog- 

tubes, an- 

eny 

tennae  and 

body 

«      « 

Kelly,  '13 

«( 

Length  of  an- 

tennal  j  oints 

«         a 

Banta 

Daphnids 

Light  reac- 

Effective in 

tions 

one  line 

"      '19 

Simoceph- 

Sex  inter- 

"Somewhat 

alus 

grades 

effective" 

Smith 

Maize 

Oil  and  pro- 

tein content 

Effective 

Tower,  '06 

Potato 

beetle 

Pigmentation 

No  effect 

May,  '17 
Zeleny,'20 

Drosophila 

Bar-eye 

<« 

«             M 

Ineffective 

after  3  to  5 

generations 

McDowell,  '15 

M 

Thoracic 

bristles 

Effective 

Homo- 

Reeves,  '16 

M 

Thoracic 

zygous 

bristles 

« 

Crosses 

Payne,  '20 

H 

Thoracic 

Effective  for 

bristles 

several  gen- 

erations 

Sturtevant, 

M 

Dichaet 

'18 

bristles 

Effective 

Pearl 

Hen 

Fecundity 

No  effect 

Castle  and 

Phillips 

Hooded  rat 

Coat  pattern 

Effective 

138 


GENETICS 


In  bacteria  it  is  possible  to  isolate  out  variants 
from  a  single  strain  but  in  none  of  the  cases  is  the 
origin  of  the  supposed  "clone"  unquestionably  from  a 
smgle  bacterium  as  it  would  need  to  be  in  order  to 
form  a  pure  line,  so  that  what  has  occurred  in  all  prob- 
ability is  the  simple  assortment  of  a  pure  line  from  a 
population. 

Among  the  protozoa,  which  reproduce  asexually  by 
fission,  much  painstaking  experiment  and  observation 


95 


FIG.  28. — Eight  pure  races  of  Paramecium.  The  actual  mean 
length  of  each  race  is  given  in  micra  below  the  corresponding 
outline.  Magnified  about  230  diameters.  After  Jennings. 


has  been  made,  notably  by  Jennings  and  various  investi- 
gators whom  he  has  inspired. 

For  example,  Jennings  found  that  Paramecia  differ 
from  each  other  in  size,  structure,  physical  character, 
and  rate  of  multiplication  as  well  as  in  the  environ- 
mental conditions  required  for  their  existence  and, 
furthermore,  that  these  differences,  in  an  hereditary 
sense,  are  "as  rigid  as  iron." 

With  respect  to  the  character  of  mean  length  he 


THE  PURE  LINE  AND  SELECTION     139 

was  able  to  isolate  eight  races,  or  pure  lines,  whose 
average  size,  drawn  to  scale,  is  shown  in  Figure  28. 

Each  of  these  pure  lines  produced  a  progeny  which 
exhibited  a  considerable  range  of  fluctuating  variation. 
The  offspring  of  pure  line  D,  for  example,  varied  from 
256  to  80  micra  x  in  length  with  an  average  of  176 
micra,  as  shown  in  Figure  29,  where  samples  of  the 
different  classes  of  variants  in  pure  line  D  are  arranged 
in  a  series. 


256  < 


Micra 


-»  80 


FIG.  29. — Diagram  of  a  single  race  (D)  showing  the  variation  in 
the  size  of  the  individuals.  Magnified  about  230  diameters. 
After  Jennings. 


A  single  representative  of  each  of  the  different  classes 
of  variants  out  of  all  the  eight  pure  lines  bred  by 
Jennings  is  shown  in  Figure  30. 

Each  horizontal  row  represents  a  single  race  or 
pure  .line,  the  average  size  of  which  is  indicated  by  the 
sign  +.  The  mean  length  of  the  entire  lot,  as  shown 
by  the  vertical  line,  is  155  micra.  The  total  number 
of  individuals  belonging  to  each  size  is  not  indicated,  but 
1 A  micron  is  1-1 000th  of  a  millimeter. 


140 


GENETICS 


in  every  horizontal  line  their  number  is  more  numerous 
near  the  average  for  that  line  and  less  numerous  at 

155 
210 
310 


FIG.  30. — Diagram  of  the  species  Paramecium  as  made  up  of  the 
eight  different  races  shown  in  Figure  28.  Each  horizontal 
row  represents  a  single  race.  The  individual  showing  the 
mean  size  in  each  race  is  indicated  by  a  cross  placed  above  it. 
The  mean  for  the  entire  lot  is  at  the  vertical  line.  The 
magnification  is  about  24  diameters.  After  Jennings. 

the   extremes,   thus   forming  the   typical   normal   fre- 
quency curves  of  fluctuating  variability. 

The  significant  fact  about  these  series  is  that  ex- 


THE  PURE  LINE  AND  SELECTION     141 

treme  individuals  selected  from  any  pure  line  do  not  re- 
produce extreme  sizes  like  themselves,  but  instead,  a 
progeny  varying  according  to  the  laws  of  chance 
around  the  average  standard  of  the  particular  line 
from  which  it  came.  Thus  quite  independently  of 
Johannsen,  Jennings  arrived  at  the  same  general  con- 
clusion, namely,  that  selection  within  a  pure  line  is  with- 
out effect. 

But  with  Difflugia,  another  protozoan  that  secretes 
for  itself  a  jug-like  shell,  Jennings,  after  a  characteris- 
tically careful  and  prolonged  study,  has  a  different 
story  to  tell.  Difflugia  proved  to  be  a  more  favorable 
form  to  study  than  Paramecium  because  it  has  numer- 
ous distinctive  shell  characters  which  are  all  inheritable 
to  a  high  degree  but  are  unchanged  by  growth  and  en- 
vironment during  the  life  of  the  individual,  although 
presenting  variations  from  parent  to  offspring. 

Jennings  selected  for  (1)  the  number  of  spines  on 
the  shell ;  (2)  the  length  of  the  spines ;  (3)  the  diameter 
of  the  shell;  (4)  the  depth  of  the  shell;  (5)  the  number 
of  teeth  surrounding  the  mouth;  (6)  the  diameter  of 
the  mouth.  In  two  families,  "one  (#303)  including 
495  descendants  of  a  single  individual,  and  the  other 
(#314)  including  1049  descendants  of  the  original 
parent,  selection  was  effectire." 

C.    PARTHENOGENETIC  PROGENY 

Parthenogenetic  animals  furnish  even  better  material 
than  unisexual  clonal  animals  for  testing  the  effective- 
ness of  selection  in  an  unmixed  line  but  here  again  the 


142  GENETICS 

conclusions  of  the  investigators  are  not  in  entire  har- 
mony. There  is  no  doubt  that  in  most  cases  selection 
within  a  parthenogenetic  line  is  futile  although  Banta's 
long  continued  observations  upon  daphnids  seem  to  fur- 
nish evidence  of  an  opposite  kind.  Particular  weight 
should  be  given  to  this  work  because  it  presents  one  of 
the  longest  pure  lines  that  ever  passed  under  the  seeing 
eye  of  a  scientist.  In  some  of  his  lines  there  have  been 
450  generations  (1921)  forming  an  unbroken  line 
extending  over  10  years'  time.  If  this  pedigree  were 
translated  into  human  generations  of  30  years  each  it 
would  make  a  period  of  13,500  years  and  would  run 
back  over  100  centuries  B.  C.  long  before  the  very 
beginnings  of  human  history.  There  is  no  doubt  that 
many  experiments  in  selection  cannot  be  considered 
decisive  because  they  concern  altogether  too  few  gen- 
erations, as  compared  with  the  time  that  has  been  at  the 
disposal  of  nature  in  accomplishing  evolutionary 
change. 

D.    HOMOZYGOUS    CEOSSES 

It  is  very  difficult  to  find  instances  among  animals 
and  plants  where  two  individuals  are  homozygous  in  all 
particulars.  The  nearest  approach  is  "identical  twins" 
which  arise  from  a  single  fertilized  egg  and  consequently 
are  more  nearly  germinally  alike,  and  can  never  cross  since 
they  are  always  of  the  same  sex. 

It  is  useful,  nevertheless,  to  consider  pure  lines  result- 
ing from  homozygous  crosses  when  limited  to  a  single 
character  rather  than  to  individuals,  for  of  this  con- 
dition there  are  numberless  instances. 


THE  PURE  LINE  AND  SELECTION     148 

a.  Tower's  Potato-Beetles 

As  an  illustration  of  the  effect  of  selection  within 
pure  lines  may  be  mentioned  Tower's  exhaustive  experi- 
ments on  the  Colorado  potato-beetle  Leptinotarsa 
decemlineata.  These  beetles  had  been  inbred  for  such  an 
extended  period  that  they  were  presumably  homozygous 
for  the  character  of  color.  Among  the  numerous  cul- 
tures which  were  under  control,  a  considerable  varia- 
tion in  color,  nevertheless,  made  its  appearance.  For 
convenience  in  classification  these  variations  were 
graded  into  arbitrary  classes  or  graduated  variants 
ranging  from  dark  to  light. 

When  a  male  and  a  female  from  the  extreme  class  at 
the  dark  end  of  the  series  were  allowed  to  breed  together, 
their  progeny  were  not  dark,  but  fluctuated  in  color 
around  the  original  average  of  the  entire  series.  The 
process  of  selecting  each  time  an  extreme  pair  of  dark 
parents  was  continued  for  twelve  generations,  as  shown 
in  Figure  31,  without  in  any  way  increasing  the  per- 
centage of  brunette  potato-beetles  in  the  progeny. 

Thus  in  a  pure  line  formed  by  the  breeding  of  two 
individuals,  alike  with  respect  to  color,  the  selection  of 
an  extreme  variant  was  quite  without  effect  in  modifying 
the  color  of  the  progeny. 

b.  Drosophila  Bristles 

Among  the  "hairs"  on  the  scutellum  of  Drosophila 
melanogaster  there  are  four  larger  hairs  or  bristles, 
as  shown  in  Figure  32. 


xn 


XI 


IX 

vin 

VII 

VI 

V 

IV 


II 


FIG.  31. — Diagram  showing  the  ineffectiveness  of  selection  through 
twelve  generations  within  a  homozygous  strain  in  the  case  of 
the  Colorado  potato-beetle  (Leptinotarsa).  In  each  genera- 
tion extremely  dark  specimens  were  selected  as  the  parents  of 
the  succeeding  generation  but  the  progeny  always  swung  back 
to  the  type.  After  Tower. 

144 


THE  PURE  LINE  AND  SELECTION     145 

These  four  bristles  are  ordinarily  strictly  accounted 
for  in  heredity  but  the  occasional  variation  in  their 
number  led  MacDowell,  and  later  others,  to  attempt  to 
establish  by  selection  a  new  style  in  these  bristly 
decorations  consisting  of  either  extra 
or  fewer  bristles.  Apparent  success 
was  the  result  in  effective  selection 
among  the  offspring  of  parents 
homozygous  for  the  single  character 
of  four  bristles. 

c.  Pearl's  Egg-laying  Hens 

In  an  experiment  extending  over 
17  years  and  which  involved  nearly 
5000    pedigreed   birds,   Pearl   tried, 
within  a  homozygous  strain,  to  select         biological       Cin- 
a  hen  that  would  produce  £00  eggs          Drawn  from 
annually    instead    of    the    ordinary     BridgesFi^  C'   J* 
number  of  125,  but  without  success. 

d.  Castle's  Hooded  Rats 

Finally  one  of  the  most  famous  selection  experiments 
on  record  is  that  of  extent  of  pigmentation,  plus  and 
minus,  in  the  hooded  rat.  This  experiment  involved 
breeding  an  average  of  nearly  twelve  rats  a  day  without 
cessation  for  eight  years  and  it  has  not  only  made  the 
Pied  Piper  of  Hamelin  roll  over  in  his  grave  but  has 
kept  biologists  busy  with  explanations  of  the  results, 
for,  like  the  four  bristles  on  Drosophila's  back,  it  ap- 


146  GENETICS 

parently  furnishes  evidence  of  modifications  of  an  he- 
reditary characteristic  through  selection  following  a 
homozygous  cross. 

Castle  succeeded  in  selecting  two  extreme  races  of 
rats  from  his  hooded  stock,  one  possessing  almost  no 
pigment  and  the  other  with  the  "hood"  so  extended 
that  it  covered  practically  the  entire  body. 

Is  then  the  germ  a  variable  thing  that  makes  it  possi- 
ble to  select  effective  differences  out  of  a  pure  line,  to 
the  discomfiture  of  Mendelians  who  build  their  house 
on  the  rock  of  constancy  of  the  germplasm,  or  can 
these  perplexing  results  be  somehow  explained? 


7.  CONCLUSION 

At  any  rate  it  would  be  gratifying  scientifically  to 
discover  one  fundamental  law  to  which  all  these  various 
cases  of  pure  line  selection  are  accountable  because  in- 
tellectual satisfaction  always  follows  upon  finding  the 
common  denominator  of  things. 

A  unifying  explanation  that  makes  a  single  harmoni- 
ous interpretation  of  these  apparently  diverse  results, 
based  on  the  idea  that  all  are  reducible  to  Johannsen's 
conception  of  the  ineffectiveness  of  selection  within  a 
pure  line,  has  perhaps  been  reached  in  the  theory  of 
modifying  genes  which  will  be  considered  in  the  next 
chapter. 

Certainly  the  pure  line  concept  is  a  very  useful  tool 
for  the  geneticist  since  with  it  the  hereditary  upset  of 
outside  germplasm  is  eliminated.  Consequently  it  is 


THE  PURE  LINE  AND  SELECTION     147 

of  the  utmost  importance  to  know  what  can  be  done  with 
this  tool.  In  any  event  the  way  of  experimentation  and 
observation  still  lies  open  and  what  remains  undiscov- 
ered makes  life  worth  living. 


CHAPTER  VII 

THE  FACTOR  HYPOTHESIS 
1.  THE  HEREDITARY  UNIT 

IN  reducing  any  body  of  facts  to  a  science,  it  is 
first  necessary  to  determine  the  underlying  units  out 
of  which  the  facts  are  made  up. 

Chemistry  was  alchemy  until  the  chemical  elements 
were  identified  and  isolated.  Histology  was  terra 
obscura  until  the  cell  theory  brought  forward  "cells" 
as  the  units  of  tissues.  In  the  same  way  there  could 
be  no  science  of  genetics  until  the  conception  was  de- 
veloped that  the  individual  is  a  bundle  of  unit  char- 
acters rather  than  a  unit  in  itself.  So  it  has  come 
about  that  geneticists  speak  of  inheritance  as  applied 
to  unit  characters  rather  than  to  individuals  as  a  whole. 

The  apparent  somatic  unit  characters,  like  the 
color  of  the  seed-coat  or  the  length  of  the  vine  in 
Mendel's  peas,  are  conditioned  by  other  intangible  but 
nevertheless  real  germinal  units  or  determiners  which 
give  rise  to  them.  Mendel  was  apparently  unaware  of 
the  existence,  in  certain  cases  at  least,  of  compound  de- 
terminers. His  experiments  led  him  to  believe  that 
each  character  depends  upon  only  a  single  determiner 
for  the  reason  that  he  worked  on  characters  severally 
belonging  to  different  parts  of  the  plant,  but  it  has 

148 


j 

: 


THE  FACTOR  HYPOTHESIS  149 

been  ascertained  within  the  last  decade  that  some  char- 
acters require  more  than  a  single  germinal  determiner 
to  bring  them  to  somatic  expression.  The  converse 
is  also  true,  namely,  that  certain  single  determiners 
may  control  more  than  one  character.  For  instance, 
the  determiner  for  gray  hair  in  rats  also  produces  a 
lighter  color  on  the  belly. 

The  idea  of  compound  germinal  determiners  for  a 
single  character  has  been  termed  the  factor  hypothesis 
of  heredity. 

Hereditary  germinal  factors,  that  may  sometimes 
need  to  combine  in  order  to  produce  a  visible  somatic 
unit  character,  are  known  as  genes  ( Johannsen). 

2.  DIFFERENT  KINDS  OF  GENES 

There  are  various  kinds  of  genes  that  bring  about  the 
visible  expression  of  unit  characters  in  various  ways. 
An  attempt  to  tabulate  the  kinds  of  genes  is  herewith 
given. 

SINGLE 

Alternative 

Allelomorphic 
Presence  or  absence 
PLURAL 

Cumulative 
Modifying 

Complementary 
Supplementary 
Lethal. 

When  genes  are  derived  from  two  parents,  as  in  all 
cases  of  sexual  reproduction,  they  are  always  in  pairs, 


150  GENETICS 

that  is,  one  from  each  parent,  and  in  the  production  of 
a  unit  character  they  may  act  in  single  or  in  several 
pairs. 

If  a  single  pair,  the  genes  may  be  interpreted  accord- 
ing to  either  the  allelomorphic  or  the  presence-or- 
absence  hypothesis.  In  the  first  instance  it  is  either 
one  thing  or  an  alternative  that  produces  the  charac- 
ter. For  example,  as  in  the  case  of  the  pea-vine,  it  is 
either  tallness  or  dwarfness.  In  the  second  instance, 
the  determiner  of  the  character  either  is  present  or 
it  is  not,  and  the  resulting  unit  character  is  dependent 
upon  which  of  these  two  possibilities  obtains.  That  is, 
applied  to  the  illustration  just  given,  if  the  hereditary 
factor  or  gene  for  tallness  is  present  the  pea-vine  will 
be  tall  but  if  there  is  no  gene  for  tallness  the  plant 
will  be  a  dwarf.  This  condition  is  expressed  by  the  term  f 
alternative  genes  and  the  operation  of  alternative 
genes  follows  in  the  typical  Mendelian  fashion  described 
in  Chapter  V. 

Under  various  kinds  of  plural  determiners  which  in- 
volve more  than  one  pair  of  genes,  cumulative  genes 
are  those  that  are  all  alike  in  their  separate  effects 
but  which,  acting  together,  alter  the  degree  of  expres- 
sion that  is  given  to  the  unit  character.  These  will  be 
more  fully  described  in  Chapter  VIII  upon  "Blending  ' 
Inheritance." 

Modifying  genes  are  those  germinal  factors  that  are 
without  effect  alone  but  which  in  conjunction  with  other 
factors  produce  an  alteration  of  those  factors.  They 
may  be  (1)  Complementary,  when  a  factor  is  added 
to  a  dissimilar  factor  in  order  that  a  particular  charac- 


THE  FACTOR  HYPOTHESIS  151 

ter  may  appear;  (2)  Supplementary,  when  a  factor  is 
added  to  a  dissimilar  factor  already  effective,  with  the 
result  that  a  character  is  modified  or  changed  in  some 
way;  (3)  Lethal,  so-called  since  they  "cause  the  early 
death  of  those  gametes  or  zygotes  in  which  such  a  fac- 
tor is  not  balanced  by  a  normal  one"  (Conklin). 

It  will  be  profitable  to  consider  a  few  illustrations 
of  the  factor  hypothesis  in  some  detail  since  it  helps  to 
explain  both  the  reappearance  of  old  types  and  the  for- 
mation of  new  ones. 

3.  COMPLEMENTARY  GENES 

In  the  course  of  numerous  breeding  experiments 
Bateson  obtained  two  strains  of  white  sweet  peas, 
Lathyrus,  which,  when  normally  self-fertilized,  each 
bred  true  to  the  white  color.  When  these  two  strains 
were  artificially  crossed,  however,  the  progeny  all  had 
purple  flowers  like  the  wild  ancestral  Sicilian  type  of 
all  cultivated  varieties  of  sweet  peas. 

Here  was  apparently  a  typical  instance  of  "rever- 
sion" which  would  have  delighted  Darwin's  heart,  but 
according  to  the  factor  hypothesis  the  true  explanation 
is  this.  The  character  of  purple  color  is  dependent 
upon  two  independent  genes  which,  though  separately 
heritable,  are  both  required  to  produce  it.  Each  of 
these  white  strains  of  sweet  peas  possesses  one  of  these 
genes  which  can  produce  colored  flowers  only  when 
united  with  its  complement,  a  proof  of  which  appeared 
upon  interbreeding  hybrid  purples  from  such  a  cross. 
In  short,  the  color  purple  depends  upon  the  action  of 


152 


GENETICS 


two  complementary  genes  that  follow  the  behavior  of  a 
dihybrid.     (See  Chap.  V,  par.  10.) 

The  gametic  formulae  for  the  two  strains  of  white 
sweet  peas   used  in  this  experiment   are  Cp   and   cP, 


©» 


CP 


© 


cP 


ep 


11 


12 


16 


A$ 


J 


FIG.  33. — Diagram  to  illustrate  the  possible  progeny  from  two 
heterozygous  purple  sweet  peas  according  to  data  from 
Bateson.  C,  color  gene  (large  circles) ;  c,  absence  of  C 
(small  circles) ;  P,  pigment  gene  (large  crosses)  ;  p,  absence 
of  P  (small  crosses).  In  the  zygotes  within  the  checkerboard 
squares  the  gametic  symbols  are  superimposed. 

respectively.  C  stands  for  a  color  gene  without  which 
no  color  can  appear,  and  c  is  the  absence  of  this  fac- 
tor, while  P  represents  a  purple  pigment  gene  which 
finds  expression  in  the  somatoplasm  only  when  taken 
together  with  the  color  gene  C.  The  small  letter  p 


THE  FACTOR  HYPOTHESIS  153 

stands  for  the  absence  of  the  purple  pigment  gene.  It 
will  be  seen  that  each  of  the  white  sweet  peas  the  for- 
mulse  of  which  are  given  above  lack  one  of  the  two 
essential  factors  for  purple  color.  When  the  two  are 
crossed,  however,  all  the  progeny  are  purple  with  the 
formula  CcPp. 

These  hybrid  sweet  peas  upon  gametic  segregation 
theoretically  produce  four  kinds  of  gametes,  CP,  Cp, 
cP,  and  cp  which  may  combine  as  any  other  dihybrid 
in  sixteen  different  ways.  In  this  case,  however,  these 
combinations  group  themselves  into  only  two  pheno- 
types,  purple  and  white,  as  indicated  in  the  accom- 
panying diagram  (Fig.  33)  in  which  C  and  c  are  repre- 
sented by  large  and  small  circles  respectively,  while 
P  and  p  are  correspondingly  indicated  by  large  and 
small  crosses.  The  gametic  symbols  are  superimposed 
to  form  the  zygotes. 

The  theoretical  expectation  here  shown  was  closely 
approximated  in  the  actual  results. 

It  may  be  noted  in  passing  that  the  seven  kinds  of 
white  sweet  peas  resulting  from  the  above  cross,  while 
phenotypically  alike,  that  is,  in  the  zygotic  symbols 
of  Figure  33,  lacking  either  the  large  circle  (color)  or 
the  large  cross  (pigment),  belong  to  three  distinct  groups 
of  genotypes  as  follows  : — 


NUMBER  OF 

ZYOOTE  IN 

FIGURE  33 

1 

2 

Without  the  pigment  gene   (large  cross) 
Without  the  color  gene  (large  circle) 

6  .  8  .  14 
11  •  12  •-  15 

3 

Without  either  pigment   (large  cross)   or  color 

(large  circle) 

16 

154 


GENETICS 


Among  the  purple  flowers   are  the  following  four 
genotypes : — 


1 

2 
3 
4 

NUMBER  OP 
ZYGOTE  IN 
FIGURE  33 

Duplex   for  both  color   (large  circle)   and  pig- 
ment (large  cross) 
Duplex  for  color  (large  circle)  but  simplex  for 
pigment   (large  cross) 
Simplex  for  color  (large  circle)  but  duplex  for 
pigment   (large  cross) 
Simplex  for  both  color   (large  circle)   and  pig- 
ment  (large  cross) 

1 
2:3 

3  *9 
4  •  7  ;  10  •  13 

4.   SUPPLEMENTARY  GENES 
A.  CASTLE'S  AGOUTI  GUINEA-PIGS 

An  illustration  of  a  supplementary  gene  that  acts 
only  in  conjunction  with  some  other  to  bring  about  a 
modification,  is  the  pattern  gene  demonstrated  by 
Castle  in  his  guinea-pigs. 

The  wild  gray,  or  "agouti,"  color  of  the  hair  of 
certain  guinea-pigs  is  due  to  the  fact  that  pigment  is 
distributed  along  the  length  of  each  hair  in  a  definite 
pattern.  The  tip  of  a  single  hair  is  black  followed 
by  a  band  of  yellow,  while  most  of  the  proximal  part 
which  is  more  or  less  concealed  by  overlapping  hairs 
is  a  leaden  color.  The  distribution  of  pigment  in  such 
a  pattern  gives  the  characteristic  gray,  or  agouti 
color  to  the  coat  when  taken  as  a  whole. 

Castle  demonstrated  the  separate  nature  and  be- 
havior of  such  a  pattern  gene  when  he  discovered  that 


THE  FACTOR  HYPOTHESIS  155 

it  is  transmitted  independently  of  pigment,  which  is 
necessary  to  bring  it  to  expression.  He  showed  that 
upon  crossing  a  solid  black  guinea-pig,  unquestionably 
possessing  pigment  but  no  "pattern,"  with  a  white 
albino  guinea-pig  having  no  pigment,  some  of  the  off- 
spring "reverted"  to  the  ancestral  agouti,  or  "pat- 
tern" type,  thus  proving  that  the  pattern  must  be 
carried  in  this  case  by  the  white  or  albino  guinea-pig 
as  a  factor  independent  of  the  color  which  is  necessary 
for  its  expression. 


Another  instance  of  the  interaction  of  supplemen- 
tary genes  is  seen  in  the  spotting  of  piebald  mice. 
Cuenot  discovered  that  such  spotting  is  due  to  the 
absence  of  a  uniformity  gene  which  if  present  causes 
color  to  be  uniformly  distributed  over  the  entire  coat. 

Both  of  these  independent  genes,  spotting  and  uni- 
formity, are  real  and  not  imaginary,  since  they  may 
be  separately  transmitted  through  albino  animals  in 
the  same  way  as  the  pattern  gene  mentioned  above, 
notwithstanding  that  in  albinos  both  are  hidden 
through  the  absence  of  pigment,  upon  the  presence  of 
which  their  visibility  depends. 

Whenever  piebald  or  spotted  animals  appear  in  a 
progeny  derived  originally  from  self-colored  stock,  it 
is  evidently  due  to  the  absence  of  such  a  "uniformity" 
gene  as  has  just  been  described. 

Galton's  theory  of  "particulate  inheritance"  (page 
94)  is  now  satisfactorily  explained  as  true  alterna- 


156  GENETICS 

tive  inheritance  in  which  the  mosaic  appearance  is 
caused  by  a  Mendelian  determiner,  in  this  instance  a 
spotting  gene  or,  in  other  words,  the  absence  of  a  gene 
for  uniformity. 


Miss  Durham,  in  her  work  with  mice,  has  demon- 
strated an  intensifying  gene,  the  absence  of  which  she 
calls  a  diluting  gene.  The  action  of  the  former  pro- 
duces, as  its  name  implies,  intensity  of  color,  while  that 
of  the  latter  serves  to  lessen  the  degree  of  intensity  in 
which  color  appears. 

These  genes  of  intensity  and  diluteness,  it  should 
be  observed,  do  not  in  any  way  correspond  to  the 
duplex  and  simplex  condition  of  a  dominant  color 
character,  either  of  which  would  straightway  appear 
if  crossed  with  an  albino.  The  factors  of  intensity 
and  dilution  of  color  are  of  an  entirely  different  na- 
ture, as  they  have  been  proven  to  be  independently 
transmissible  through  albinos  where  a  color  character 
could  not  appear  because  of  the  absence  of  pigment. 

The  following  illustration  of  this  kind  of  supple- 
mentary genes  taken  from  Miss  Durham's  experiments 
will  serve  to  make  the  case  clear.  The  symbols  em- 
ployed are: — 

B  =  black  pigment  which  masks  brown,  or  chocolate. 
6  =  the  absence  of  B,  consequently  chocolate. 
I  =  intensity  gene. 

i  =  dilution  gene  or  absence  of  intensity. 
C  —  a  complementary  color  gene  acting  with  P. 
p  =  a  complementary  pigment  gene  acting  with  (7. 
BICP  =  black. 


THE  FACTOR  HYPOTHESIS 


157 


BiCP  — blue  or  maltese   (dilute  black). 

bICP  =  chocolate. 

biCP  =  silver-fawn  (dilute  chocolate). 


The  results  of  crossing  the  hybrids  formed  from  the 
combinations  indicated  at  the  left  in  the  table  below  are 
shown  at  the  right  where  the  expectation  is  given  in 
parentheses  after  the  actual  results. 


BLACK 
(BICP) 

BLUE 
(BiCP) 

CHOCO- 
LATE 
(bICP) 

SILVER- 
FAWN 
(MCP) 

Black  (BICP)    X  Silver-fawn  (MOP) 
Blue   (BiCP)    X   Chocolate   (6/OP).. 
Blue  (BiCP)    X  Silver-fawn   (biCP)  . 

9(9) 
42(45) 
0(0) 

4(3) 
16(15) 
33(36) 

3(3) 
14(15) 
0(0) 

2(1) 
8(5) 
12(12) 

It  will  be  seen  that  the  actual  results,  even  when 
such  small  totals  are  concerned,  approximate  very 
closely  the  expectation  and  are  entirely  consistent. 


D.  CASTLE'S  BROWN-EYED  YELLOW  GUINEA-PIGS 

Furthermore  Castle  has  shown  that  in  guinea-pigs 
there  is  an  independent  gene  for  extension  of  pigment 
distinct  from  the  uniformity  gene  already  mentioned. 
The  absence  of  this  extension  gene  ("restriction")  is 
manifested  by  a  lack  of  black  or  brown  pigment  every- 
where except  in  the  eyes  and  to  a  slight  extent  in  the 
skin  of  the  extremities,  while  the  distribution  of  yellow 
is  wholly  unaffected  by  it. 

That  such  "extension"  and  "restriction"  genes 
really  exist,  is  proven  in  the  following  way: — 

When  a  brown  (chocolate)  guinea-pig  is  crossed 
with  an  ordinary  black-eyed  yellow  one,  the  young  are 
all  black  pigmented,  but  by  cross-breeding  these  hybrid 


158 


GENETICS 


young  four  varieties  are  obtained  in  the  next  genera- 
tion, viz.,  black,  brown,  black-eyed  yellow,  and  brown- 


10 


bE    \Be 

Black 


14 


'be  \  I  Be 

'.  Yellow 


IbE 


BE  \\bE 

Black 


BellbE 

Black 


11 


bE  \\bE 

Chocolate 


15 


be    I  bE 

Chocolate 


|6e 


Be  \\be 

Black-eyed  Yellow 


12 


bE\\b* 

Chocolate 


16 


be\  \be 

3rown-eyed  Yellow 


FIG.  34. — Diagram  to  illustrate  the  origin  of  a  brown-eyed  yellow 
guinea-pig  from  two  heterozygous  black  parents,  based  upon 
Castle's  experiments.  The  gene  for  yellow  (Y)  is  present  in 
every  gamete  and  is  consequently  duplex  in  every  zygote  but 
is  hidden  whenever  the  gene  B  is  present.  B,  black  pigment 
hiding  brown  or  chocolate;  b,  chocolate  (absence  of  B)  ;  E, 
extension  of  B  over  the  entire  body  hiding  Y;  e,  restriction 
of  B  to  eyes  alone  thus  exposing  Y  over  the  entire  body. 

eyed    yellow,    the    latter    a    variety    unknown    before 
Castle's  experiment  in  breeding  was  made. 

For   the    sake    of   clearness    the    formation    of   the 
brown-eyed  yellow  is  shown  above  in  Figure  34. 


THE  FACTOR  HYPOTHESIS  159 

Symbols 

B  =*  black  pigment,  hiding  brown  or  chocolate. 
b  =  absence  of  B,  or  chocolate. 
Y  =  yellow  pigment,  hidden  by  B. 
E  =  extension  of  B  over  entire  body,  hiding  Y. 
e  =  restriction  of  B  to  eyes   alone,  thus  exposing   Y  over  the 

entire  body. 

O  =  complementary  color  gene  acting  with  P  to  produce  color. 
P  =  complementary  pigment  gene  acting  with  C  to  produce  color. 
(The  genes  C  and  P  may  be  omitted  for  the  sake  of  sim- 
plicity, since  they  are  present  in  each  instance.) 

First  Cross 

"Extended"  chocolate  (bEY)  X  black-eyed  yellow 
(BeY)  =  black  (BbEeYY). 

Second  Cross 

When  these  cross-breds  are  mated  with  each  other, 
they  each  form  four  kinds  of  gametes,  BEY,  BeY, 
bEY,  and  beY,  which  unite  into  sixteen  theoretical 
genotypic  possibilities,  some  of  which  are  unlike  (Fig.  34). 
These  fall  into  four  phenotypes,  nine  black  (BEY),  three 
black-eyed  yellow  (BeY),  three  chocolate  (bEY),  and 
one  brown-eyed  yellow  (beY).  The  actual  results  in 
Castle's  experiments  gave  all  four  kinds  in  close  nu- 
merical agreement  with  this  expectation.  The  action 
of  extension  and  restriction  genes  is,  therefore,  plainly 
a  case  of  Mendelian  dihybridism  in  which  two  inde- 
pendent pairs  of  alternative  characters  are  concerned. 

E.   BABBIT    PHENOTYPES 

Perhaps  no  better  application  of  the  factor  hy- 
pothesis, so  far  as  supplementary  genes  are  concerned, 
may  be  found  than  in  the  case  of  the  color  of  rabbits. 


160 


GENETICS 


There  are  many  varieties  of  rabbits  with  respect  to 
color,  particularly  among  domesticated  races.  These 
varieties  are  now  quite  explainable  by  the  factor  hy- 
pothesis, as  indicated  in  the  table  below.  The  sixteen 
kinds  of  rabbits  there  catalogued  have  been  obtained  by 

THE  FACTOR  HYPOTHESIS  APPLIED  TO  COLORS  or  RABBITS 


CONSTANT 
GENES 

ALTERNATIVE 
GENES 

GAMETIC 
FORMULA 

PHENOTYPIC  CHARAC- 
TER WHEN  CROSSED 
WITH  THE  SAME  KIND 
OP  GAMETIC 
COMBINATION 

1 

2 

3 

4 

5 

6 

7 

8 

Br 

B 

Y 

0 

E 

I 

U 

A 
a 

AUIEC[YBBr] 

Gray 

UIEC  [YBBr] 

Black 

u 

A 

duIEC  [YBBr] 

Gray  spotted 

a 

cwIEC  [YBBr 

Black  spotted 

i 

I 

U 

A 

AUiEC[YBBr 

31ue-gray 

a 

aUiEC  [YBBr 

Blue  (Maltese) 

u 

A 
a 

AuiEC  [YBBr 

Blue-gray  spotted 

auiEC    [YBBr 

Blue  spotted 

e 

U 

A 
a 

AUIeC  [YBBr 

JYellow    (with    white 
[     belly  and  tail) 

aUIeC  [YBBr 

f  Sooty  yellow  (with 
yellow   belly   and 
tail) 

u 

A 

AuleC  [YBBr 

Yellow  spotted 

a 

auleC    [YBBr 

Sooty  yellow  spotted 

i 

U 

A 

AUieC  [YBBr 

Cream 

a 

aUieO    [YBBr 

Pale  sooty  yellow 

u 

A 
a 

AuieC    [YBBr 

Cream  spotted 

auieC     [  YBBr 

Pale   sooty  yellow 
spotted 

THE  FACTOR  HYPOTHESIS  161 

Castle  and  other  experimental  breeders,  as  well  as  many 
of  the  albino  types  that  would  double  this  list  if  c,  or 
the  gene  for  absence  of  color,  should  be  substituted  for 
C,  the  presence  of  color,  in  column  4  of  the  table  on 
page  160. 

Explanation  of  Symbols  in  the  Foregoing  Table 

Br  =  a  gene  acting  on  C  to  produce  brown  pigmentation. 

B  =  a  gene  acting  on  C  to  produce  black  pigmentation. 

Y  =  a  gene  acting  on  C  to  produce  yellow  pigmentation. 

The  three  genes,  Y,  B,  Br,  are  present  in  every  rabbit 
gamete  and  up  to  date  have  not  been  separable  as  inde- 
pendent unit  characters,  although  they  have  been  sepa- 
rated out  in  guinea-pigs  and  mice.  There  are  no  brown 
rabbits,  because  black  always  goes  linked  with  brown 
covering  the  brown  factor.  Yellow  rabbits  result,  as 
explained  below,  through  the  action  of  factor  e. 

C  =  a  common  color  gene  necessary  for  the  production  of  any 
pigment.  It  was  discovered  in  1903  by  Cu6not. 

c  =  the  absence  of  C  which  results  in  albinos,  regardless  of 
whaterer  pigment  gene  may  be  present.  By  changing  C 
to  c,  sixteen  kinds  of  albinos  would  be  added  to  this 
catalogue,  an  addition  of  one  phenotype  and  sixteen 
genotypes,  all  looking  alike  but  breeding  differently. 

E  =  a  gene  governing  the  extension  of  black  and  brown  pig- 
ment, but  not  of  yellow. 

e  =  the  absence  of  extension  or  restriction  of  black  and  brown 
pigment  to  the  eyes  and  the  skin  of  the  extremities  only, 
while  yellow  remains  extended  and  visible.  Demonstrated 
by  Castle  in  1909. 

7  =  an  intensity  gene  which  determines  the  degree  of  pigmenta- 
tion. It  can  be  transmitted  independently  of  C  through 
an  albino.  Discovered  by  Bateson  and  Durham  in  1906. 

i  =  the  absence  of  intensity  or  dilution.  Dilute  black  =  blue. 
Dilute  yellow  =  cream.  Dilute  gray  =  blue-gray. 

U  =  a  gene  for  uniformity  of  pigmentation  or  "self-color"  dis- 
covered by  Cuenot  in  1904. 

u  =  the  absence  of  uniformity  which  results  in  spotting  with 

white. 

A  =  a  pattern  gene  for  agouti,  or  wild  gray  color,  which  causes 
the  brown  and  black  pigments  to  be  excluded  from  cer- 
tain portions  of  each  hair,  resulting  in  the  gray  coat. 
When  present  in  the  rabbit,  it  is  also  associated  with 
white  or  lighter  color  on  the  under  surfaces  of  the  tail 
and  belly.  It  was  demonstrated  by  Castle  in  1907. 

a  =  the  absence  of  the  agouti  or  pattern  gene. 


162  GENETICS 

F.   THE   KINDS  OF   GRAY  RABBITS 

Each  of  the  apparent  kinds  of  gray  rabbits  indicated 
in  the  foregoing  table  may  be  made  up  of  various  geno- 
types. For  instance,  there  are  thirty-two  different 
genotypes,  each  of  which  is  pheno typically  a  gray 
rabbit.  The  zygotic  formula  for  each  of  these  thirty- 
two  possibilities  is  displayed  in  the  next  table,  and  it 
will  be  seen  that  these  range  all  the  way  from  rabbits 
homozygous  in  all  their  variable  characters  (No.  1) 
to  those  homozygous  in  none  (No.  32). 

The  progeny  of  these  various  types  of  gray  rabbits 
when  inbred  will  consequently  vary  from  the  pure  gray, 
as  in  No.  1,  to  a  gray  from  which  sixteen  possible 
types  of  young  may  be  expected  as  in  No.  32. 

Up  to  the  time  when  Castle's  paper  upon  the  factor 
hypothesis 1  was  published  in  1909,  nine  genotypic 
kinds  of  gray  rabbits  had  been  obtained  in  his  experi- 
ments, whose  genotypic  formulae  correspond  to  the 
following  numbers  in  the  list:  1,  3,  6,  10,  13,  20,  22, 
28,  29. 

5.  LETHAL  GENES 

Among  mammals,  as  shown  by  Cuenot  and  confirmed 
by  Little,  homozygous  or  pure  yellow  mice  are  un- 
known although  yellow  individuals  have  long  been  ex- 
ploited by  fanciers.  In  other  words,  all  kinds  of  yel- 
low mice  behave  as  if  heterozygous  or  simplex  with 
respect  to  yellow  color,  for  when  any  two  yellow  mice 

1  "Studies  of  Inheritance  in  Rabbits."  Carnegie  Institution 
Publications,  No.  114,  1909.  W.  E.  Castle  in  collaboration  with 
Walter,  Mullenix  and  Cobb. 


THE  FACTOR  HYPOTHESIS  163 

THE  KINDS  OF  GRAY  RABBITS  (Color  only) 


NUM- 

BER OF 

CjrENOTYPE 

HET- 

PHENOTYPES 

ZYQOTIC  FORMULA 

EROZY- 
GOTIC 

When  inbred,  these  kinds  are  produced 

FAC- 

TORS 

1 

AAUUIIEECC  (YBBr]  [YBBr] 

None 

X 

2 

AAUUIIEECc  [YBBr][YBBr] 

One 

X 

X 

8 

AAU  UIIEeCC   (  YBBr]  [  YBBr] 

One 

X 

x 

4 

AAUUHEECC  [YBBr]  (YBBr] 

One 

X 

X 

5 

AAUuIiEECC  [YBBr][YBBr] 

One 

x 

x 

6 

AaUUIIEECC  [YBBr]  [YBBr] 

One 

X 

X 

7 

AAUUIIEeCc     [YBBr](YBBr] 

Two 

X 

X 

X 

8 

AAU  UliEECc    [  YBBr]  (  YBBr] 

Two 

x 

x 

X 

9 

A  A  UuIIEECc    (  YBBr]  (  YBBr] 

Two 

x 

X 

X 

10 

AaU  UIIEECc    [  YBBr]  [  YBBr] 

Two 

X 

X 

X 

11 

AAUUIiEeCC    (YBBr]  [YBBr] 

Two 

x 

x 

X 

X 

12 

AA  UuIIEeCC    [YBBr]  [YBBr] 

Two 

x 

X 

x 

x 

13 

AaU  UIIEeCC    (  YBBr]  (  YBBr] 

Two 

X 

x 

X 

X 

14 

AAUuIiEECC    (YBBr]  [YBBr] 

Two 

X 

X 

x 

X 

15 

AaUUIiEECC    (YBBr]  (YBBr] 

Two 

X 

X 

x 

X 

10 

AaUuIIEECC    [YBBr][YBBr] 

Two 

X 

X 

X 

X 

17 

Aa  UuIiEECC     (  YBBr]  (  YBBr] 

Three 

X 

X 

X 

X 

X 

x 

X 

X 

18 

Aa  UuIIEeCC     [  YBBr]  [  YBBr] 

Three 

X 

X 

x 

X 

X 

X 

X 

X 

19 

Aa  UuIIEECc     [  YBBr]  (  YBBr] 

Three 

X 

X 

X 

X 

X 

20 

AaU  UliEeCC     (  YBBr]  [  YBBr] 

Three 

X 

X 

X 

X 

X 

X 

X 

x 

21 

AaUUIiEECc     [YBBr]  [YBBr] 

Three 

x 

X 

X 

X 

x 

22 

AaUUIIEeCc     [YBBr][YBBr] 

Three 

X 

X 

X 

X 

x 

23 

AAUuIiEeCC     [YBBr][YBBr] 

Three 

X 

x 

X 

x 

X 

X 

X 

x 

24 

A  A  UuIIEeCc     [  YBBr]  (  YBBr] 

Three 

x 

X 

X 

X 

x 

25 

AAUuIiEECc     (YBBr]  [YBBr] 

Three 

X 

x 

x 

x 

x 

26 

A  A  U  UliEeCc     (YBBr]  [  YBBr] 

Three 

x 

X 

X 

X 

X 

27 

AAUuIiEeCc      (YBBr]  (YBBr] 

Four 

x 

X 

X 

X 

x 

X 

x 

X 

X 

28 

AaUUIiEeCc      (YBBr]  (YBBr] 

Four 

x 

X 

X 

X 

x 

X 

x 

X 

X 

29 

Aa  UuIIEeCc       [  YBBr]  [  YBBr] 

Four 

X 

X 

X 

X 

x 

X 

X 

X 

X 

30 

AaUuIiEECc      (YBBr]  [YBBr] 

Four 

X 

X 

X 

X 

x 

X 

X 

X 

X 

31 

Aa  UuIiEeCC      [  YBBr]  [  YBBr] 

Four 

X 

X 

X 

X 

x 

x 

X 

X 

X 

X 

X 

X 

X 

x 

X 

x 

32 

AaUuIiEeCc       [YBBr][YBBr] 

Five 

x 

X 

X 

X 

x 

X 

X 

x 

X 

X 

X 

X 

X 

X 

X 

xx\ 

0 

a 

1 

0 

I 

1 

f 

|  Blue  graj 

w 

g 

|  Blue  gra? 

w 

I" 

~ 

I 

|  Yellow  si 

i 

1 

P3 

5" 

? 

80 

B 

I 

1 

"S 

1 

0 

I 

I 

I 

1 

i 

164  GENETICS 

are  bred  together  they  produce  a  certain  percentage 
of  recessives  lacking  yellow  which  would  not  happen  if 
they  were  pure  yellow.  Hundreds  of  yellow  individuals 
have  been  tested  but  they  always  produce  in  addition 
to  yellow  some  non-yellow,  that  is,  black,  brown  or  gray 
individuals.  That  the  non-yellow  individuals  are  re- 
cessive is  shown  by  the  fact  that  when  inbred,  they 
produce  no  yellow  offspring,  therefore,  yellow  is  domi- 
nant. 

In  a  Mendelian  monohybrid  cross,  as  has  been  pre- 
viously pointed  out,  the  expectation  is  that  in  the  sec- 
ond generation  one  fourth  of  the  offspring  will  be  re- 
cessives (DR  X  DR  =  DD  +  2  DR  +  RR),  but  when 
yellow  mice  are  bred  together,  the  percentage  of  re- 
cessives approximates  one-third  instead  of  one-fourth. 
Little,  in  a  total  of  over  1200  young  produced  by  yel- 
low parents,  obtained  almost  exactly  two-thirds  yel- 
low.    This  apparent  exception  to  the  Mendelian  ratio 
finds  an  explanation,  however,  when  it  is  assumed  that 
D   (yellow)   is  a  lethal  gene  when  present  m  duplex 
(DD)  form.     The  DDs  drop  out  entirely  which  leaves 
the  proportion  approximately  two  DRs  and  one  RR. 
This  supposition  is  further  supported  by  the  fact  that 
the  litters  of  young  from  yellow  mice  are,  on  an  aver- 
age, only  three-fourths  as  large  as  normal  litters  of 
mice,  which  is  exactly  what  would  be  expected  if  one- 
fourth  of  the  possible  gametic  combinations  (DD)  fail 
to  produce  offspring.    Moreover,  evidence  of  the  death 
m  utero  of  the  pure  yellow  mice  has  been  produced  by 
Ibsen  and  Stiegleder,  '17. 


THE  FACTOR  HYPOTHESIS  165 

Morgan  and  his  associates  have  demonstrated  the 
existence  of  over  twenty  different  lethal  factors  in 
Drosophila  which  when  inherited  from  both  parents 
not  only  prevent  the  development  of  any  unit  charac- 
ters but  also  doom  the  individual  to  death.  Only 
heterozygotes  for  such  lethals,  who  receive  the  death 
warrant  from  one  parent  alone,  may  escape  and  hand 
on  this  fatal  determiner. 

In  plants  lethal  genes  have  been  demonstrated  by 
Baur  in  snapdragons  and  by  Lindstrom  in  maize.  In 
these  instances  the  lethal  factor  is  a  lack  of  chlorophyll 
which  is  not  fatal  if  inherited  from  a  single  parent 
because  the  deficiency  is  supplied  by  a  gene  for 
chlorophyll  from  the  other  parent,  but  when  the  lack 
comes  from  both  parents  it  produces  a  seedling  unable 
to  survive. 

Recently'G.  H.  Shull  has  demonstrated  the  existence 
of  two  balanced  recessive  lethal  factors  in  one  pair  of 
the  fourteen  chromosomes  in  (Enothera,  one  pair  pro- 
ducing a  lethal  effect  in  the  zygote,  the  other  pair 
destroying  the  gametes.  This  fact  explains  many  of 
the  hitherto  confusing  ratios  obtained  in  breeding  this 
classical  plant. 

"Such  lethal  factors  modify  the  expected  Mendelian 
ratios  and  greatly  complicate  the  study  of  genetics, 
but  they  do  not  destroy  its  fundamental  principles, 
indeed  when  properly  understood  they  furnish  one  of 
the  strongest  proofs  of  the  truth  of  the  factorial 
theory  of  heredity"  (Conklin). 


166  GENETICS 

6.  MODIFYING  GENES  AND  SELECTION 

The  recognition  of  modifying  genes  has  furnished 
an  explanation  for  the  apparent  effectiveness  of  selec- 
tion within  a  pure  line  without  assuming  germinal 
inconstancy. 

The  gene  itself,  like  that  producing  the  hooded  pat- 
tern of  Castle's  rats,  is  constant  but  it  is  accompanied 
by  a  halo  of  modifying  genes  likewise  constant  which 
have  no  somatic  expression  except  when  the  original 
factor  for  hooded  pattern  is  present.  These  modifying 
genes  are  simply  potential  increasers  or  diminishers  of 
the  hooded-pattern  gene.  In  the  absence  of  the  pattern 
gene  there  is  nothing  to  increase  or  diminish  and  con- 
sequently there  is  no  way  to  demonstrate  the  modifying 
factors.  They  are  not  imaginary  things,  however,  for 
their  separate  existence  and  transmissibility  have  been 
demonstrated  from  many  sides.  What  selection  within 
the  progeny  of  the  homozygous  cross  of  hooded  rats 
or  bristly  flies  accomplishes  is  simply  the  elimination 
or  addition  of  either  plus  or  minus  modifying  genes, 
according  as  the  attempt  is  being  made  to  increase 
or  decrease  the  hooded  pattern  of  pigmentation  or  the 
number  of  bristles. 

If  this  explanation  stands  the  test  of  further  investi- 
gation then  we  are  still  dealing  in  heredity  with  con- 
stant dependable  units,  such  as  the  chemist  finds  in  his 
elements,  and  it  may  be  said  that  all  genetic  roads 
lead  to  the  Rome  of  gene-constancy. 

However,  it  is  well  to  remember  that  Darwin  did  not 
revolutionize  the  concept  of  evolution  until  he  broke 


THE  FACTOR  HYPOTHESIS  167 

down  the  idea  of  constancy  of  species  and  that  geology 
did  not  come  into  its  own  until  Lyell  substituted  for 
constancy  the  molding  hand  of  incessant  change. 

No  doubt  there  is  a  substratum  of  unity  underlying 
all  of  these  processes  and  Mendelians,  therefore,  may 
still  retain  their  constant  characters  undismayed  and 
have,  at  least,  the  three  following  ways  left  by  means 
of  which  it  is  possible  to  get  results  in  selection: — 

(1)  By  the  isolation  of  pure  lines  if  the  stock  is 
hybrid  (heterozygous)  ; 

(2)  By  the  elimination   or  addition  of  modifying 
genes  if  the  stock  is  pure  (homozygous) ; 

(3)  By  mutation  of  the  genes. 

It  should  be  repeated  that  change  by  mutation  does 
not  beg  the  question  of  constancy  of  the  genes.  A 
mutation  is  not  a  changed  gene.  It  is  the  substitution 
of  an  entirely  different  onet 


CHAPTER  VIII 

BLENDING  INHERITANCE 

1.  RELATIVE  SIGNIFICANCE  OF  DOMINANCE  AND 
SEGREGATION 

OF  the  three  fundamental  principles  which  underlie 
"Mendel's  law,"  namely,  segregation,  independence  of 
unit  characters,  and  dominance,  the  principle  of  domi- 
nance has  been  found  to  hold  true  in  a  surprising  num- 
ber of  cases  and  in  relation  to  very  diverse  organisms, 
notwithstanding  the  fact  that  its  universal  application 
is  by  no  means  assured. 

Mendel  himself  noted  certain  exceptions  to  the  law 
of  dominance,  and  his  followers  have  pointed  out  with 
increasing  emphasis  that  it  is  subject  to  many  modifi- 
cations. It  is  now  understood,  indeed,  that  segrega- 
tion, not  dominance,  is  the  most  essential  factor  in  the 
Mendelian  scheme. 

2.  IMPERFECT  DOMINANCE 

It  frequently  occurs  that  dominance  is  so  imperfect 
that  a  heterozygous,  or  simplex,  dominant  may  be  dis- 
tinguished at  once  by  simple  inspection  from  a  homo- 
zygous,  or  duplex,  dominant,  whereas  the  test  of  cross- 
ing with  a  recessive  is  necessary  whenever  dominance 
is  complete,  as  has  been  previously  explained.  The 

168 


BLENDING  INHERITANCE  169 

single  dose  of  the  determiner  in  such  a  case  has  plainly, 
then,  less  phenotypic  effect  than  a  double  dose. 

There  are  many  instances  of  imperfect  dominance 
among  flowering  plants.  Correns*  red  and  white  four- 
o'clocks  with  pink  offspring  (p.  106)  is  a  case  in  hand. 

A  classic  illustration  of  imperfect  dominance  among 
animals  is  the  "blue  Andalusian  fowl,"  the  hereditary 
behavior  of  which  is  illustrated  below  (Fig.  35).  It 
will  be  seen  that  when  two  blue  Andalusian  fowls, 

Andalusian  Andalusian 


j       i  r       i 

Black       Andalusian  Andalusian     Splashed  White 


\ 


I:   .  I  II 

Black    Andalusian     Andalusian  Spl. White  Spl.White 


Andalusian 


FIG.  35.— The  heredity  of  the  blue  Andalusian  fowl,  an  illustra- 
tion of  "imperfect  dominance." 
I 

characterized  by  a  mottled  plumage,  are  bred  together, 
they  produce  three  kinds  of  offspring  in  the  ratio  of 
1  :  2  : 1.  Twenty-five  per  cent  are  clear  black,  50  per 
cent  are  blue  Andalusian,  and  25  per  cent  are  white 
"splashed"  with  black.  Both  the  black  and  the 
splashed  white  fowls  from  this  cross  prove,  upon  fur- 
ther breeding,  to  be  homozygous,  while  the  blue  Anda- 
lusian itself  is  heterozygous  and  can,  therefore,  never 
be  made  to  breed  true.  In  order  to  produce  100  per 
cent  of  blue  Andalusian  chicks,  it  is  necessary  simply 
to  cross  a  splashed  white  with  a  black  Andalusian. 
There  is  nothing  in  this  case  to  indicate  whether  the 


170  GENETICS 

black  or  the  splashed  white  should  be  regarded  as  the 
homozygous  dominant,  since  dominance  is  imperfect. 
In  either  case  the  heterozygous  blue  Andalusian  is  at 
once  evident  in  the  first  filial  generation  without  further 
crossing. 

A  similar  case  of  imperfect  dominance  is  furnished 
by  the  roan  color  of  cattle  which  results  when  red  and 
white  are  crossed.  If  two  roans  are  mated,  they  pro- 
duce red,  roan,  and  white  offspring  in  the  proportion 
of  1  :  2  : 1,  thus  showing  that  roan  is  a  heterozygous 
character  in  which  the  dominance  of  red  is  imperfect. 

Even  in  cases  of  apparently  perfect  dominance  it  is 
sometimes  possible  by  close  inspection  to  detect  differ- 
ences between  a  pure  dominant  (DD),  Figure  17,  and 
a  heterozygous  dominant  (DR)  when  a  superficial  ex- 
amination is  not  sufficient  to  distinguish  them. 

Morgan  cites  a  Drosophila  cross  between  "ebony" 
and  "sooty"  wings  wherein  the  F2  generation  ranges 
from  ebony  to  sooty  in  an  inseparable  transition  but 
it  proves,  nevertheless,  to  be  of  three  classes  in  the 
proportion  of  1  :  2  : 1,  as  further  breeding  tests  show. 


3.  DELAYED  DOMINANCE 

A  character  which  is  really  dominant  is  sometimes 
so  late  in  manifesting  itself  in  the  individual  growth 
of  the  offspring  that  it  may  properly  be  termed  a 
delayed  dominant. 

Dark-haired  individuals  often  do  not  acquire  their 
definitive  hair  color  until  adult  life,  and  it  is  common 
knowledge  that  the  eyes  of  an  infant  for  a  consider- 


BLENDING  INHERITANCE  171 

able  period  provoke  no  little  speculation  among  ador- 
ing relatives  as  to  "whose  eyes"  they  are. 

According  to  Davenport,  when  a  white  Leghorn 
fowl  is  crossed  with  a  black  Leghorn,  white  being 
dominant  in  this  case,  chicks  are  produced  that  are 
white  with  black  flecks  in  their  plumage.  These  black 
flecks,  however,  disappear  at  the  time  of  the  first  molt. 
The  complete  dominance  of  white  is,  therefore,  simply 
delayed. 

4.  "REVERSED"  DOMINANCE 

In  certain  instances  there  seems  to  be  a  reversal  of 
dominance,  as  may  be  illustrated  by  Lang's  results  with 
snails  (Helix).  *He  has  proven  in  his  experiments  that 
red  snails  are  generally  dominant  over  yellow  snails, 
although  in  certain  cases  there  is  apparently  an 
exception  to  the  rule,  for  snails  with  yellow  shells 
dominate  those  with  red  shells. 

Davenport  has  shown  too  that  although  extra  toes 
are  usually  dominant  over  the  normal  number  in  poul- 
try, yet,  in  something  like  20  per  cent  of  the  cases,  the 
normal  number  is  dominant. 

It  sometimes  occurs  that  a  character  which  is  domi- 
nant in  one  species  may  be  recessive  in  another.  Horns 
are  dominant  in  sheep,  but  recessive  in  cattle.  White 
color  is  recessive  in  rodents  and  sheep,  but  dominant 
in  most  poultry  and  in  pigs. 

Again  Morgan  describes  a  Drosophila  that  possesses 
a  gene  for  abnormally  banded  abdomen  which  does  not 
come  to  somatic  expression  unless  the  flies  are  supplied 
with  fresh  food  and  a  proper  amount  of  moisture. 


172  GENETICS 

When  the  food  becomes  dried  up  and  there  is  a  mini- 
mum of  moisture  the  banding  on  the  abdomen  disap- 
pears. Here  is  a  reversal  of  dominance  but  the  gene 
itself  is  not  affected  since  the  same  flies  which  are 
hybrid  with  respect  to  the  character  of  banding,  show 
a  difference  according  to  the  environment  of  food  and 
moisture,  in  the  amount  of  banding  given  expression. 
Notched  margin  in  leaves  is  dominant  in  nettles  and 
recessive  in  the  celandine.  Again  a  negative  character 
may  be  the  dominant  one  in  a  pair  of  allelomorphs. 
For  example,  the  bob-tail  of  the  Manx  cat  is  dominant 
over  the  ordinary  long  tail  of  the  cat;  the  reduced 
number  of  three  digits  in  guinea-pigs  is  dominant  over 
four  digits ;  the  polled  condition  is  dominant  over  horns 
in  cattle;  the  rumpless  fowl  is  dominant  over  the  fowl 
with  a  rump,  and  brachydactyly  in  man,  that  is,  fingers 
or  toes  with  only  two  joints  each,  is  dominant  over 
the  three- jointed  arrangement. 

5.  POTENCY 

Davenport  seeks  to  explain  modifications  in  typical 
dominance  as  variations  in  the  potency  of  determiners. 
He  defines  potency  as  follows:  "The  potency  of  a 
character  may  be  defined  as  the  capacity  of  its  germi- 
nal determiner  to  complete  its  entire  ontogeny." 

That  is,  if  the  potency  of  a  determiner,  for  some 
reason,  is  insufficient,  there  may  be  either  an  incom- 
plete or  delayed  manifestation  of  the  character  in 
question,  or  it  may  fail  entirely  to  develop. 

The  variations  of  potency  may  be  grouped  into  three 


BLENDING  INHERITANCE  173 

general  categories  according  to  the  degree  of  their 
manifestation ;  namely,  total  potency,  partial  potency, 
and  failure  of  potency. 

A  further  word  of  explanation  for  each  of  these 
three  kinds  of  potency  seems  desirable  at  this  point. 

A.    TOTAL    POTENCY 

This  is  complete  Mendelian  dominance  in  which 
even  the  heterozygotes  produced  by  a  simplex  dose  of 
a  character  are  indistinguishable  phenotypically,  that 
is,  by  inspection,  from  the  homozygotes  produced  by  a 
duplex  dose  of  the  same  character.  It  is  as  if  a  single 
bottle  of  black  ink  poured  into  a  jar  of  water  was 
just  as  effective  as  two  bottles  of  ink,  in  forming  an 
opaque  fluid. 

Even  in  the  cases  of  apparently  complete  dominance, 
however,  refined  methods  of  examination  or  analysis 
may  make  it  possible  to  distinguish  the  duplex  from 
the  simplex  condition  without  recourse  to  breeding. 
Darbishire  has  shown,  in  the  case  of  Mendel's  smooth 
and  wrinkled  peas,  that  the  two  kinds  of  smooth  prog- 
eny from  the  Fj  hybrid  upon  microscopic  examination 
show  a  difference  in  their  starch  grains  that  indicates 
at  once  which  is  homozygous  and  which  is  heterozy- 
gous. Moreover,  in  the  power  of  absorption,  hybrid 
smooth  peas  (DR)  are  intermediate  between  their 
pure  dominant  smooth  (DD)  and  pure  recessive 
wrinkled  (RR)  parents. 

Blakeslee  has  demonstrated  a  chemical  method  of 
distinguishing  unseen  genetic  differences  in  the  appar- 


174  GENETICS 

ently  similar  flowers  of  the  black-eyed  susan  (Rud- 
beckia  hirta).  When  placed  in  a  solution  of  KOH, 
the  yellow  cones  of  one  kind  turn  a  purplish-black, 
while  the  other  kind  turns  red. 

B.    PARTIAL    POTENCY 

Partial  potency  covers  all  cases  of  incomplete 
dominance,  such  as  those  of  the  four-o'clock  (Mira- 
bilis)  and  blue  Andalusian  fowls,  where  a  simplex  dose 
of  a  determiner  does  not  produce  the  same  visible  effect 
as  a  double  dose. 

The  dominant  prickly  jimson  weed  (Datura),  when 
crossed  with  a  recessive  glabrous  variety  of  the  same 
plant,  produces  cross-breds  in  the  first  generation 
which  show  only  a  few  prickles  (Bateson)  (Baur), 
following  the  law  of  partial  potency. 

Banded  and  uniformly  colored  snails  also,  when 
crossed  together,  produce  snails  with  shells  showing 
only  a  pale  banding  (Lang). 

Numerous  further  instances  of  incomplete  domi- 
nance could  be  cited. 

C.    FAILURE    OF    POTENCY 

If  for  any  reason  a  determiner  fails  to  accomplish 
its  possibilities  in  whole  or  in  part,  then  the  character 
in  question  may  never  become  evident,  and  the  result, 
so  far  as  appearances  go,  is  the  same  as  if  it  was  a 
recessive  lacking  the  determiner  entirely. 

That  the  failure  of  potency  is  not  identical  with  the 
absence  of  a  determiner  can  usually  be  demonstrated 


BLENDING  INHERITANCE  175 

by  further  breeding,  because  dominants  failing  in  po- 
tency, which  are  either  of  the  formula  DD  or  DR, 
may,  if  bred  inter  se,  give  a  various  progeny  among 
which  the  dominant  character  D  is  likely  to  become 
manifest  again,  while  recessives,  of  the  formula  RR, 
on  the  contrary,  will  invariably  give  offspring  which 
all  agree  in  the  entire  absence  of  the  character  in 
question. 

Davenport  cites  an  extreme  case  of  failure  of  po- 
tency in  one  of  two  rumpless  cocks  from  the  same  blood. 
The  character  of  rumplessness  is  due  to  an  inhibitor 
of  tail  development.  That  these  two  cocks  both  pos- 
sessed this  character  was  demonstrated  by  the  entire 
absence  of  any  tail  in  either  case.  The  inhibiting  de- 
terminer for  tail  growth  was  so  weak  in  cock  No.  117, 
however,  that,  to  quote  Davenport's  exact  words:  "In 
the  heterozygote  the  development  of  the  tail  is  not 
interfered  with  at  all,  and  even  in  extracted  dominants 
it  interfered  little  with  tail  development,  so  that  it 
makes  itself  felt  only  in  the  reduced  size  of  the  uro- 
pygium  and  in-bent  or  shortened  back.  But  in  No.  116 
the  inhibiting  determiner  is  strong.  It  develops  fully 
in  about  47  per  cent  of  all  the  heterozygotes  and  in 
extracted  dominants  may  produce  a  family  in  all  of 
which  the  tail's  development  is  inhibited." 

Here  were  two  birds  of  the  same  blood,  phenotyp- 
ically  alike  and  presumably  genotypically  alike,  which 
because  of  an  individual  difference  in  the  potency  of 
the  determiner  for  rumplessness  produced  quite  differ- 
ent results  in  their  offspring  although  bred  to  precisely 
the  same  array  of  hens. 


176  GENETICS 

6.  BLENDING  INHERITANCE 

In  the  instances  of  imperfect  dominance  given  above, 
where  the  progeny  of  unlike  parents  present  an  inter- 
mediate condition,  it  is  found  that,  upon  cross-breed- 
ing these  offspring,  segregation  into  the  grandparental 
types  occurs  just  as  truly  as  in  instances  of  complete 
dominance. 

In  poultry,  for  example,  when  Cochins,  which  are 
"booted,"  and  Leghorns,  which  are  clean-shanked,  are 
crossed,  booting  of  an  intermediate  grade  of  four  re- 
sults, on  a  scale  in  which  ten  represents  complete  boot- 
ing, and  zero,  no  booting  or  clean  shank  (Davenport). 
The  character  of  booting  and  its  alternative  absence, 
however,  segregate  out  in  true  Mendelian  fashion  when 
these  hybrids  are  subsequently  crossed  together.  It  is 
evident  that  dominance  plays  only  a  secondary  role  in 
such  cases,  and  that  the  all-important  factor  is  segre- 
gation. 

Are  there,  then,  any  cases  where  true  fusion  of 
hereditary  parental  traits  occurs,  in  other  words, 
where  segregation  in  the  second  filial  generation  does 
not  appear?  Does  the  "melting-pot  of  cross-breed- 
ing" ever  "melt"  the  characters  thrown  into  it? 

It  was  formerly  believed  that  diverse  parents  gener- 
ally produce  intermediate  offspring,  and  that  this 
intermediate  condition  continues  without  any  segrega- 
tion at  all  in  the  form  of  "blending  inheritance,"  but 
within  the  last  decade  apparent  cases  of  blending  in- 
heritance have  been  thrown  out  of  court  one  after  the 
other  by  the  Mendelians.  Bateson,  in  an  inaugural 


BLENDING  INHERITANCE  177 

address  at  Cambridge  University  in  1908,  stated  that 
what  was  once  believed  to  be  the  rule  has  now  become 
the  exception.  He  goes  on  to  say:  "One  clear  excep- 
tion I  may  mention.  Castle  finds  that  in  a  cross  be- 
tween the  long-eared  lop  rabbit  and  a  short-eared 
breed,  ears  of  intermediate  length  are  produced;  and 
that  these  intermediates  breed  approximately  true." 

Let  us  examine  this  "one  clear  exception"  a  little 
more  closely. 

7.  THE  CASE  OF  RABBIT  EAES 

As  a  typical  example  of  blending  inheritance  in 
rabbit  ears  the  following  case  may  be  cited : — 

A  female  Belgian  hare  with  an  ear-length  of  118  mm. 
was  crossed  with  a  male  lop-eared  rabbit  with  an  ear- 
length  of  210  mm.  The  average  of  these  ear-lengths 
is  164  mm.  Five  offspring  from  this  pair  had  ear- 
lengths,  when  adult,  approximating  this  average  as 
follows:  170,  170,  166,  156,  170,  of  which  two  were 
females  and  three  were  males.  When  from  this  litter 
one  of  the  females  measuring  170  mm.  in  ear-length 
was  subsequently  crossed  with  her  brother  having  an 
ear-length  of  166  mm.,  two  litters  were  produced  in 
which  the  individuals  when  adult  attained  ear-lengths 
of  170,  166,  168,  160,  172,  and  168  mm.  These 
results  are  represented  diagrammatically  in  Figure  36. 

This  illustration  is  typical  of  many  other  breeding 
experiments  made  by  the  same  investigators  1  upon  the 

1  Castle,  in  collaboration  with  Walter,  Mullenix  and  Cobb. 
"Studies  of  Inheritance  in  Rabbits."  Carnegie  Institution  Publi- 
cations, Washington,  No.  114^  1909. 


178 


GENETICS 


Offspring  of  1  and  7 


Offspring  of  3  and  5 


$  1  3  I  5  5 


FIG.  36. — A  case  of  three  generations  of  ear-length  in  rabbits. 
0-6,  average  ear-length  of  the  first  filial  generation  (FJ. 
a'-b',  average  ear-length  of  the  F2  generation  derived  from 
1  and  7.  Data  from  Castle,  in  collaboration  with  Walter, 
Mullenix  and  Cobb. 

ear-length  of  rabbits  which  included  70  different  litters 
of  rabbits  containing  341  individuals.  In  none  of  these 
experiments  could  the  blend  in  the  second  filial  genera- 


BLENDING  INHERITANCE  179 

tion  be  called  perfect,  but  it  may  at  least  be  said  that 
evidence  of  segregation,  that  is,  a  return  to  one  or  the 
other  of  the  parental  types,  was  much  less  apparent 
than  evidence  of  blending. 

Furthermore,  crosses  were  made  in  which  lop  ears 
of  various  fractional  lengths  were  obtained  as  desired, 
including  |,  i,  f,  i,  f ,  f ,  and  £  lengths.  Not  one  of 
these  fractional  lengths  apparently  segregated  in  sub- 
sequent generations  after  the  Mendelian  fashion,  but 
all  bred  approximately  true. 

Moreover,  ears  of  one  half  lop  length,  for  instance, 
were  obtained  in  three  ways:  first,  by  crossing  full- 
length  lops  with  short-eared  rabbits  as  indicated  in  the 
first  cross  of  the  case  cited  above;  second,  by  crossing 
one  half  lop  lengths  together,  demonstrated  by  the 
second  cross  in  the  illustrative  case  given,  and  third, 
by  mating  J  and  f  lop  lengths.  Theoretically,  -J  and  J- 
as  well  as  f  and  f  lop  lengths  would  also  produce 
\  lop  lengths,  for  in  all  of  the  crosses  that  were  made 
the  length  of  ear  behaved  in  a  blending  fashion. 

These  results  were  based,  not  upon  a  single  measure- 
ment of  each  specimen,  which  might  be  open  to  con- 
siderable error,  but  upon  daily  measurements  from  the 
time  the  rabbits  were  two  weeks  old  until  their  ears 
ceased  to  grow  at  about  twenty  weeks.  The  growth 
curves  drawn  from  these  daily  measurements  showed 
continually  an  intermediate  or  blending  condition  in 
progeny  derived  from  diverse  parents. 

A  Mendelian  explanation  of  this  apparently  excep- 
tional case  of  blending  inheritance  has  been  suggested 
by  Lang  based  upon  the  result  of  Nilsson-Ehle's  dis- 


180  GENETICS 

coveries    while    breeding   wheats    at    the   Agricultural 
Experiment  Station  of  Svalof  in  Sweden. 


8.  THE  NILSSON-EHLE  DISCOVERY 

Nilsson-Ehle,  1907,  found  in  breeding  together 
different  strains  of  wheat  that  a  certain  wheat  with 
brown  chaff  crossed  with  a  white-chaffed  strain  yielded 
only  brown-chaffed  wheat  in  the  first  generation. 
These  heterozygous  or  hybrid  brown-chaffed  wheats 
when  crossed  with  each  other  produced,  not  the  ex- 
pected proportion  of  three  brown  to  one  white,  but 
•fifteen  brown  to  one  white.  This  was  not  explainable 
as  the  chance  result  of  a  single  cross,  but  was  the  con- 
clusion drawn  from  fifteen  different  crosses,  all  of  the 
same  strains,  that  yielded  a  total  progeny  of  1410 
brown-chaffed  to  94  white-chaffed  plants,  which  hap- 
pens to  be  exactly  the  proportion  of  fifteen  to  one. 

In  other  experiments  it  was  discovered  that  although 
dominant  red-kerneled  strains  of  wheat  crossed  with 
white-kerneled  varieties  usually  gave  the  three-to-one 
proportion  upon  segregation  in  the  second  filial  genera- 
tion, yet  one  particular  strain  of  red-kerneled  Swedish 
wheat  in  the  second  generation  gave  approximately 
sixty-three  red  to  one  white-kerneled  strain. 

The  explanation  of  these  two  unexpected  results  is 
this.  In  the  case  of  brown-chaffed  wheat  there  are  two 
independent  determiners  for  the  character  of  brown 
color,  and  these  simply  follow  the  Mendelian  laws  for 
a  dihybrid,  while  in  the  case  of  the  red-kerneled  -wheat 
there  are  three  independent  determiners  for  the  charac- 


BLENDING  INHERITANCE 


181 


ter  of  red  color,  each  of  which  is  able  to  give  red  color 
to  the  wheat.  Taken  together,  these  three  red  color 
determiners  behave  cumulatively,  following  the  law  of  a 
trihybrid. 

For  example,  if  a  brown-chaffed  wheat  with  the 
formula  BB',  in  which  B  and  JB'  each  represent  a 
brown  -  chaffed 
factor,  is 
crossed  with  a 
white  -  chaffed 
wheat  of  the 
formula  bb'9  in 
which  b  and  b' 
each  represent 
the  absence  of 
B  and  B'  re- 
spectively, then 
all  the  progeny 
of  this  cross 
will  be  brown- 
chaffed,  having 
the  zygotic  for- 
mula BB'W. 
When  upon  ma- 
turation the  gametes  form  out  of  the  germ-cells  from 
such  hybrids,  the  following  four  combinations  are  pos- 
sible, and  no  others :  BB',  Bb',  bB',  bb'.  These  repre- 
sent, therefore,  the  possible  gametes  present  in  each  sex 
of  the  first  filial  generation,  and  upon  intercrossing 
they  may  combine  into  sixteen  possible  zygotes  to  form 
the  second  filial  generation,  as  shown  in  Figure  37. 


BB1 

Bb' 

bB' 

bb' 

BB1 

Bb1 

bB' 

bb1 

BB1 

BB' 

BB1 

BB' 

BB' 

© 

® 

® 

® 

BB' 

Bb' 

bB' 

bb' 

Bb' 

Bb1 

Bb' 

Bb' 

Bb' 

® 

® 

® 

© 

BB1 

Bb1 

bB' 

66' 

bB' 

bB' 

bB' 

bB' 

bB' 

® 

® 

® 

© 

BB' 

Bb1 

bB' 

66' 

bb' 

bb' 

bb' 

bb' 

66' 

® 

© 

SL 

(o) 

FIG.  37. — Diagram  of  the  possible  combina- 
tions in  the  F2  generation  of  brown-chaffed 
wheat  according  to  experiments  of  Nilsson- 
Ehle.  B  and  B'  are  cumulative  factors  for 
the  brown-chaff  character;  b  and  b'  denote 
the  absence  of  B  and  B'  respectively. 


182  GENETICS 

The  numbers  in  the  squares  in  Figure  37  indicate  how 
many  times  a  brown  determiner  is  present  in  each 
zygote.  It  will  be  seen  that  only  one  out  of  the  sixteen 
possibilities  lacks  a  brown-chaff  factor,  and  this  one  will 


43210 

Number  of  doses  of  the  brown  determiner 

FIG.  38.— The  distribution  of  the  sixteen  possibilities  resulting 
when  two  similar  determiners  (brown-chaff)  act  together  as 
a  dihybrid. 

consequently  produce  only  white  chaff,  while  the  re- 
maining fifteen  possibilities,  each  of  which  has  at  least 
a  single  determiner  for  brown,  will  all  yield  brown 
chaff. 

The   brown-chaff    factor,    moreover,    is    present    in 


« 


BLENDING  INHERITANCE 


183 


varying  doses  among  these  fifteen  possibilities,  as  indi- 
cated by  the  numbers  in  the  squares.  It  is  evident, 
therefore,  that  several  shades  of  brown  will  be  repre- 


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FIG.  39. — Diagram  to  illustrate  Nilsson-Ehle's  case  of  trihybrid 
red  wheat.  The  large  screw-heads  each  represent  a  single 
determiner  for  the  red  character.  The  small  screw-heads 
symbolize  the  absence  of  the  red  character,  or  white.  The 
number  in  each  square  indicates  how  many  doses  of  the  "red" 
determiner  is  present.  For  further  explanation  see  text. 


sented  depending  upon  the  number   of  doses   of  the 
brown  determiner  in  each  instance. 

Figure  38  shows  how  these  different  shades  of  brown 
arrange  themselves  in  the  manner  of  a  frequency  curve 
of  fluctuating  variation  with  the  greatest  number  in 


GENETICS 

the  halfway  class  and  the  least  numbers  at  the  two 
extremes.  In  this  instance  six  out  of  sixteen  individu- 
als of  the  second  generation  theoretically  present  a 
perfect  "blend"  between  the  original  brown-  and  white- 
chaffed  grandparents,  although  complete  segregation 
has  actually  occurred. 

The  same  explanation  holds  true  as  displayed  in 
Figure  39  for  the  trihybrid  case  of  red-  and  white- 
kerneled  wheats  in  which  only  one  white-kerneled  to 
sixty-three  red-kerneled  individuals  appear  in  the  sec- 

000  +  0  e  o=0o0e0o 

Pure  red     -f-     white       =3        Hybrid  red 

FIG.  40.— The  result  of  crossing  white  wheat  with  trihybrid  red 

wheat. 

ond  filial  generation.  The  number  of  red  determiners 
in  each  zygote  is  indicated  by  the  figure  at  the  bottom 
of  each  square.  The  large  screw-head  symbols  with 
vertical,  horizontal  and  diagonal  slots  each  represent 
an  independent  determiner  for  red  kernel,  while  the 
small  screw-heads  symbolize  the  absence  of  each  of 
these  determiners,  or  white  kernel.  When  the  pure 
strain  of  red-kerneled  wheat  is  crossed  with  a  pure 
strain  of  white-kerneled  wheat,  the  first  generation  is 
all  a  heterozygous  red  of  a  somewhat  lighter  shade 
than  the  original  pure  red  strain  as  shown  diagram- 
matically  in  Figure  40. 

When  plants  of  this  heterozygous  sort  are  crossed 
together,  they  yield  plants  producing  red-kerneled  and 
white-kerneled  wheats  in  the  proportion  of  sixty-three 
to  one.  The  sixty-three  kinds  of  red  wheats  are  of 


BLENDING  INHERITANCE 


185 


# 


varying  degrees  of  redness  and  may  be  classified  after 
the  manner  of  fluctuating  variations  with  the  greatest 
number  of  kinds  at  the 
intermediate  degree  be- 
tween pure  red  and  pure 
white.  (See  Figure  41.) 

In  order  to  test  whether 
the  sixty-four  kinds  of 
wheats  produced  in  the 
second  filial  generation,  as 
theoretically  displayed  in 
Figure  39,  really  contain 
separable,  though  indis- 
tinguishable, determiners 
for  red-kernel,  Nilsson- 
Ehle  produced  families  of 
the  third  filial  generation 
by  self-crossing  plants  of 
the  second  generation.  It 
was  to  be  expected  that,  if 
these  hybrid  wheats  of  the 
second  generation  carried 
one,  two,  three,  or  more 
determiners  for  a  red  ker- 
nel as  the  theoretical 
tables  in  Figures  39  and 
41  demand,  their  progeny 
would  be  distributed  with 
reference  to  the  number  of 
red-  and  white-kerneled 
individuals,  in  the  follow- 
ing ratios : — 


5      4 


3 


# 


0 


FIG.  41.— The  distribution  of 
the  sixty-four  possibilities  in 
the  F2  generation  when  three 
similar  determiners  act  to- 
gether as  a  trihybrid. 


186  GENETICS 

3  red  to  1  white  when  1  heterozygous  determiner  for  red 

is  present. 

15  red  to  1   white  when  2  "1  heterozygous     deter- 

63  red  to  1   white  when  3  >      miners  for  red  are 

All  red  to  no  white  when  4  or  more  J       present. 

Among  seventy-eight  sample  families  of  the  third 
generation  inbred  to  test  this  theoretical  conclusion, 
the  actual  results  were: — 

8  families  giving  the  ratio  of  3  red  to  1   white. 
15   families  giving  the  ratio  of  15  red  to   1   white. 

5  families  giving  the  ratio  of  63  red  to  1   white. 
50  families  giving  the  ratio  of  all  red  to  no  white. 

It  has  been  actually  demonstrated  therefore,  in  the 
case  of  this  particular  strain  of  wheat:  (1)  that  the 
factors  producing  red  kernels  are  several  in  number; 
(2)  that  they  act  independently  of  each  other  in 
heredity;  (3)  that  these  several  independent  factors 
segregate;  and  (4)  that  any  one  red  factor  acting 
alone  produces  a  "red"  result. 

The  Nilsson-Ehle  principle  of  cumulative  determiners 
has  been  confirmed  in  America  by  East  in  a  masterly 
series  of  breeding  experiments  upon  maize. 

In  connection  with  the  Nilsson-Ehle  principle,  it 
will  be  seen  that  the  possible  number  of  intergrades 
between  the  two  extremes  increases  rapidly  as  the  num- 
ber of  duplicate  determiners  increases.  Thus  with  six 
duplicate  determiners  for  the  same  character  present, 
the  ratio  of  possible  dominants  to  recessives  in  the 
second  filial  generation  would  be  4095  to  1.  The  re- 
appearance of  this  single  recessive  among  4095  domi- 


BLENDING  INHERITANCE  187 

nants  would  be  extremely  unlikely,  and  it  might  easily 
be  mistaken  for  a  mutation  or  a  freak.  Apparent 
blends  of  all  intermediate  degrees,  however,  would  be 
sure  to  appear.  Yet  these  are  not  blends  in  the 
"melting-pot"  sense  at  all,  but  strictly  cases  of 
Mendelian  dominance  and  segregation. 

9.  THE  APPLICATION  OF  THE  NILSSON-EHLE  EXPLA- 
NATION TO  THE  CASE  OF  RABBIT  EAR-LENGTH 

The  so-called  blending  rabbit  ears,  along  with  other 
similar  cases,  can  now  be  made  to  fall  into  line,  as 
pointed  out  by  East  and  by  Lang,  with  the  Mendelian 
law  of  segregation. 

If  we  assume  that  the  long  ear  of  the  lop  rabbit  has 
only  three  independent  but  equal  determiners  for  excess 
length,  the  case  becomes  one  of  Mendelian  trihybridism 
with  cumulative  factors,  which  works  out  like  Nilsson- 
Ehle's  red-kerneled  wheat  in  the  following  manner: — 

In  general  the  average  for  full  lop  ear-length  may 
be  placed  at  220  mm.  and  for  the  ordinary  short- 
eared  rabbit 1  at  100  mm.  The  difference,  or  the  excess 
length  of  the  lop  ear,  is  120  mm.,  which,  according  to 
the  trihybrid  formula,  corresponds  to  the  six  doses  of 
the  character  symbolized  in  the  upper  left-hand  square 
in  Figure  39  by  six  large  screw-heads,  three  coming 
from  each  parent  respectively.  If  all  of  these  inde- 
pendent determiners  are*  equal  as  regards  excess  ear- 
length,  each  factor  would  represent  an  excess  of  20  mm. 

JNot  the  Belgian  hare,  as  cited  in  the  illustration  given  in 
Figure  36.  The  Belgian  hare  has  typically  a  somewhat  longer 
ear  than  the  ordinary  short-eared  rabbit. 


188 


GENETICS 


above  the  normal  ear-length  found  in  short-eared  rab- 
bits, that  is, — 

220  mm.  —  100  mm. 

— =  20  mm. 
6 

When  according  to  this  computation  a  lop  (20 
mm.  X  6  +  100  mm.  =  220  mm.)  and  a  pure  short- 
eared  rabbit  (20  mm.  X  0  +  100  mm.  =  100  mm.) 
are  crossed,  if  imperfect  dominance  occurs,  which  is  a 
very  common  phenomenon,  it  is  true  that  the  offspring 
might  present  a  "blended"  appearance.  If  now  these 
cross-breds  of  the  first  generation  prove  to  be  tri- 
hybrids  with  respect  to  excess  ear-length,  there  would 
be  sixty-four  possibilities  in  their  progeny  segregating 
out  just  as  in  the  red-kerneled  wheat. 

These  possibilities   would  be  arranged  in   the  fol- 
lowing frequencies: — 


NUMBER  OF  EXCESS 
EAR-LENGTH 
DETERMINERS 

NUMBER  OF  CASES 

OCCURRING  OUT  OF   64 

TOTAL  LENGTH  IN 
MILLIMETERS  OF 
EARS  RESULTING 

6 

1 

220 

5 

6 

200 

4 

15 

180 

3 

20 

160 

2 

15 

140 

1 

6 

120 

0 

1 

100 

Since  the  average  litter  among  rabbits  is  about  five, 
the  chances  that  these  five  rabbits  will  breed  true  to 
their  hybrid  parents  and  form  a  perfect  blend  between 
their  grandparents  is  20  out  of  64,  while  the  chance 
of  their  being  like  either  grandparent  is  only  one  out 
of  64. 


BLENDING  INHERITANCE  189 

It  should  be  noted  further  that  50  out  of  64,  or  77 
per  cent,  of  these  hybrids  of  the  second  filial  genera- 
tion would  have  an  ear-length  between  140  and  180, 
thus  approximating  a  "blend"  closely  enough  to  be  so 
classified  upon  a  casual  inspection. 

If  it  should  be  found,  moreover,  that  excessive  ear- 
length  in  rabbits  is  due  to  more  than  three  duplicate 
determiners,  the  possibilities  of  getting  anything  but 
an  apparent  blend  would  be  much  decreased. 

Furthermore,  the  fact  that  the  fractional  ear- 
lengths  of  the  hybrid  rabbits  in  Castle's  experiments 
bred  approximately  true  in  the  second  and  subsequent 
filial  generations,  may  also  be  explained  by  the  Nilsson- 
Ehle  hypothesis. 

For  example,  half  lop  lengths,  according  to  this 
explanation,  are  those  with  three  doses  of  the  deter- 
miner for  excess  ear-length.  It  follows  that  the 
progeny  of  two  rabbits  each  carrying  three  doses  of 
a  determiner  will  likewise,  after  the  reduction  during 
the  maturation  of  the  germ-cells,  have  three  doses  of 


the  determiner 


It  would  be  interesting  to  breed  rabbits  having  ears 
of  one-eighth  lop  length  in  which,  according  to  the 
foregoing  hypothesis,  there  presumably  would  be 
present  only  a  single  determiner  for  excess  ear-length, 
with  ordinary  short-eared  rabbits  having  no  excess 
ear-length,  in  order  to  see  if  the  expected  Mendelian 
three-to-one  proportion  for  a  monohybrid  would  ap- 
pear in  the  progeny. 


190  GENETICS 

10.  HUMAN  SKIN  COLOR 

Finally,  in  man  the  skin-color  of  mulattoes,  in  hy- 
brids between  blacks  and  whites,  has  often  been  men- 
tioned as  a  case  of  blending  inheritance  since  mulattoes 
are  commonly  supposed  to  produce  mulattoes  when 
they  mate  together  or  a  blending  degree  of  color  when 
they  mate  with  some  one  whose  shade  of  color  is  unlike 
their  own. 

This  matter  has  been  carefully  and  extensively 
studied  by  Davenport  and  Danielson  l  who  came  to  the 
conclusion  that  the  pure-blooded  negro  of  the  West 
Coast  of  Africa  possesses  two  pairs  of  duplicate  genes 
for  black  pigmentation  (A ABB)  which,  though  sepa- 
rately heritable,  are  cumulative  in  effect.  The  corre- 
sponding formula  for  black  pigmentation  in  a  normal 
white  is  adbb.  When  black  (AABB)  and  white  (aabb) 
are  crossed,  the  formula  for  the  mulatto  will  be  AaBb 
in  which  half  the  total  amount  of  black  pigment  of  both 
parents  is  present. 

The  result  of  crossing  two  mulattoes  is  shown  by 
checkerboard  diagram  in  Fig.  42. 

The  figures  in  the  corners  of  the  squares  indicate 
the  total  amount  of  black  pigment  in  each  case  upon 
the  supposition  that  A  =  19,  B  =  1 6,  #  =  2  and 
b  =  1,  these  values  being  determined  by  the  color-top 
method  described  by  Davenport  and  Danielson. 

In  the  table  on  page  191  is  a  classification  of  these 
possibilities  according  to  the  amount  of  black  pigment 

1  Heredity  of  Skin  Color  in  Negro  and  White  Crosses.  Pub. 
No.  188  of  the  Carnegie  Inst.  of  Washington, 


BLENDING  INHERITANCE 


191 


present,  and  in  Figure  43  is  a  graphic  representation 
of  the  numerical  chances  for  skin-color  in  spite  of 
segregation  when  two  mulattoes  mate. 


AB 


Ab 


aB 


ab 


AB 


Ab 


aB 


ab 


AB 

a  b 


70 


55 


53 


Ab 


AB 


55 


Ab 


40 


- 


aB 
AB 


53 


••• 


•£ 

/Ab 


21 


ab 


a  b 
B 


23 


21 


6 


FIG.  42. — Checkerboard  to  show  the  different  expected  shades  of 
black  color  in  the  possible  offspring  of  two  mulattoes.  A  = 
18;  B  =  17;  a  =  2;  b  =  l  percent  of  black  pigment.  Data 
from  Davenport  and  Danielson. 


The  case  thus  is  similar  to  that  of  Nilsson-Ehle's 
brown-chaffed  wheat  already  described,  so  that  the 
possible  range  of  offspring  of  a  mulatto  pair  is  all  the 
way  from  black  to  white.  Theoretically,  any  one  of 
five  degrees  of  pigmentation,  including  the  extremes 
of  black  and  white,  may  be  expected.  The  chances 


192 


GENETICS 


A  CLASSIFICATION  OF  HYBRID  SKIN  COLORS  ON  THE  BASIS  OF  THE 
FACTOR  HYPOTHESIS 


Genes 
All  absent 

Gametic 
Formula 

Color 

Relative 
Fre- 
quency 

Range  of 
Percent  of 
Pigment  in 
Offspring 

Popular  Names 
(Jamaica) 

aabb 

White 

1:16 

0-11 

Pass-for-white 
Mustifino 
Mustifee 
Octaroon 

One  present 

Aabb 
aaBb 

Light 

4:16 

12-25 

Quadroon 

Two  present 

AAbb 
AaBb 
aaBB 

Medium 

6:16 

26-40 

Mulatto 

Three  present 

AABb 
AaBB 

Dark 

4:16 

41-55 

Mangro 
Sambo 

All  four  present 

AABB 

Black 

1:16 

56-78 

Negro 

which  any  one  of  these  five  degrees  of  color  has  of 
reappearing  in  a  child  of  mulatto  parents  is  indicated 
in  Fig.  43.  It  is  evident  that  there  is  more  likelihood, 
in  point  of  fact  six  chances  out  of  sixteen,  that  a 
child  from  mulatto  parents  will  be  mulatto  than  any- 
thing else,  and  this  expectation  ordinarily  agrees  with 
the  realization,  but  there  are  four  chances  out  of  six- 
teen that  it  will  be  either  darker  or  lighter  than  its 
parents  and  one  chance  out  of  sixteen  that  it  will  be 
as  dark  or  as  light  as  its  black  or  white  grandparents. 

Davenport  and  Danielson  show  several  illuminating 
photographs  of  large  families  of  children  from  mulatto 
parents  in  which  a  manifest  inequality  of  color  shade 
among  the  different  children  is  apparent  as  would  be 
expected  according  to  this  explanation. 

Blending  inheritance,  then,  is  probably  nothing  more 


BLENDING  INHERITANCE 


193 


than  Mendelian  alternative  inheritance  in  which  two  or 
more  similar  genes,  duplicate  or  cumulative,  are  con- 
cerned. One  explanation  instead  of  two  or  three, 


White 


Light 


Dark 


Black 


14641 

FIG.  43. — Diagram  to  show  the  expectation  of  color  and  its  fre- 
quency in  the  cross  between  two  mulattoes. 

therefore,  is  sufficient  to  dispose  of  an  array  of  appar- 
ently diverse  phenomena. 

"We  may  therefore  conclude,"  says  Conklin,  "that 
the  Mendelian  law  of  heredity,  especially  as  regards 
segregation  of  inheritance  factors,  is  of  universal  oc- 
currence— that  there  is  no  other  type  of  inheritance." 


CHAPTER  IX 

OLD  TYPES  AND  NEW 

1.  THE  DISTINCTION  BETWEEN  REVERSION  AND 
ATAVISM 

THERE  are  two  ways  in  which  types  of  animals  or 
plants  that  are  different  from  the  present  ones  may 
be  conceived  to  arise,  namely,  by  the  reappearance  of 
old  types  and  by  the  formation  of  new  ones.  In  the 
reappearance  of  old  types  a  distinction  may  be  drawn 
between  reversion  and  what  has  been  termed  atavism. 

Atavism,  or  "grandparentism,"  may  be  defined  as 
skipping  a  generation  with  the  result  that  a  particular 
character  in  the  offspring  is  unlike  the  corresponding 
character  in  either  parent,  but  instead,  resembles  the 
character  in  one  of  the  grandparents. 

In  reversion,  on  the  contrary,  a  character  reappears 
which  has  not  been  manifest  perhaps  for  many  gen- 
erations, although  it  was  actually  present  in  some 
remote  ancestor.  J.  Arthur  Thomson's  definition  of 
reversion  is :  "All  cases  where  through  inheritance  there 
reappears  in  an  individual  some  character  which  was 
not  expressed  in  his  immediate  lineage,  but  which  had 
occurred  in  a  remoter,  but  not  hypothetical,  ancestor." 

This  distinction  between  atavism  and  reversion  be- 
comes clearer  by  illustration. 

194 


OLD  TYPES  AND  NEW 


195 


If  heterozygous  brown-eyed  individuals  mate,  there 
is  one  possibility  in  four  that  their  offspring  will  have 
blue  eyes  unlike  their  own,  but  like  the  two  blue-eyed 

Grandfather      Grandmother       Grandfather 


Grandmother 


Homozygote       Hobiozygote 
.  Duplex^  NuUiplex 


Heterozygote 
Simplex 


Homozygote 
Duplex 


Heterozygote 
Simplex 


Homozygote 
Nulliplex 

FIG.  44. — Three  generations  of  a  Mendelian  monohybrid.  The 
outlines  represent  the  somatoplasms  with  the  phenotypic 
character  on  the  outside.  The  black  symbols  inclosed  within 
the  somatoplasm  stand  for  the  germplasm  in  the  form  of 
gametes.  The  short  dotted  arrows  indicate  the  relation  be- 
tween germplasm  and  somatoplasm.  The  long  dotted  arrows 
indicate  possible  recombinations  of  germplasms. 

grandparents.  Such  a  blue-eyed  child  would  be  an 
instance  of  atavism.  The  explanation  of  this  appar- 
ently inconsistent  hereditary  behavior  is  perfectly 
simple  in  the  light  of  the  Mendelian  ratios,  as  shown 


196  GENETICS 

diagrammatically  in  Figure  44,  in  which  the  circles 
represent  the  blue-eyed  and  the  squares  the  brown- 
eyed  character. 

This  figure  also  illustrates  what  typically  occurs  in 
the  formation  of  Mendelian  monohybrids  of  the  first 
and  second  filial  generations.  The  squares  are  symbols 
for  the  dominant  characters,  while  the  circles  are  sym- 
bols for  the  recessive  characters.  When  the  two  are 
superimposed,  the  circle  recedes  from  view.  The  large 
outside  figures  indicate  the  somatoplasm,  therefore  the 
phenotype.  The  small  inclosed  figures  indicate  the 
germplasm,  therefore  the  genotype.  The  short  dotted 
arrows  indicate  what  it  is  that  determines  the  somato- 
plasm in  each  case,  while  the  long  dotted  arrows  show 
what  possible  recombinations  of  germplasms  can  be 
made.  Child  No.  4  is  an  "extracted  recessive"  derived 
from  dominant  parents,  but  with  one  recessive  grand- 
parent on  each  side.  It  is  a  case  of  "atavism,"  or 
taking  after  the  grandparent.  Notice  that  atavism 
can  occur  only  by  alternative  inheritance. 

To  quote  Davenport:  "In  the  majority  of  cases 
atavism  is  a  simple  reappearance  in  one  fourth  of  the 
offspring  of  the  absence  of  a  character  due  to  the 
simplex  nature  of  the  character  in  both  parents." 

An  illustration  of  reversion  would  be  the  reappear- 
ance of  the  ancestral  jungle-fowl  pattern  in  domestic 
poultry  or  of  the  slaty  blue  color  of  the  ancestral 
rock-pigeon  among  buff  and  white  domestic  pigeons, 
for  the  ancestral  character  or  characters  in  this  type 
of  hereditary  behavior,  as  said  before,  reappear  only 
after  a  lapse  of  many  generations. 


OLD  TYPES  AND  NEW  197 

2.  FALSE  REVERSION 

"Around  the  term  'reversion/ "  Bateson  observes, 
"a  singular  set  of  false  ideas  have  gathered  them- 
selves." In  proof  of  this  statement  there  may  be  cited 
at  least  five  categories  of  apparent  reversion  which 
properly  ought  not  to  be  classed  as  true  reversion. 

A.    ARRESTED    DEVELOPMENT 

Feeble-mindedness  is  not  reversion  to  ancestral 
forms  of  less  intelligence,  but  an  instance  of  arrested 
development  when,  for  some  reason,  the  individual  fails 
to  accomplish  his  normal  cycle  of  development. 

Likewise  harelip  in  man  is  not  a  case  of  reversion  to 
rabbit-like  ancestors  in  which  harelip  is  the  normal 
condition,  but  it  is  ordinarily  due  to  an  arrest  or  fail- 
ure of  certain  embryonic  steps  essential  to  the  develop- 
ment of  the  usual  form  of  human  lip. 

B.   VESTIGIAL   STRUCTURES 

These  are  the  vanishing  remains  of  characters  that 
were  formerly  of  significance.  They  do  not  represent 
something  latent  that  is  now  reappearing,  for  they 
have  never  yet  disappeared  phylogenetically,  and  con- 
sequently they  cannot  be  regarded  as  true  reversions. 

The  muscles  under  the  scalp  which  enable  those 
persons  possessing  them  to  wiggle  the  ears;  the  pala- 
tine ridges  in  the  roof  of  the  mouth  of  many  babies 
and  some  adults  which  resemble  the  ridges  in  the  roof 
of  a  cat's  mouth ;  the  vermiform  appendix,  a  necessary 


198  GENETICS 

part  of  the  digestive  apparatus  of  many  animals  but 
fraught  so  often  with  evil  consequences  to  man;  these 
and  scores  of  similar  characters,  which,  taken  together, 
make  man  in  the  eyes  of  the  comparative  anatomist  a 
veritable  old  curiosity  shop  of  ancestral  relics,  are  the 
last  traces  of  characters  which  formerly  had  a  sig- 
nificance in  some  of  man's  forbears.  Having  lost  their 
usefulness,  these  structures  still  hang  on  to  the  ana- 
tomical household  as  pensioners.  They  have  not  been 
recalled  from  the  past,  but  have  always  been  with  us, 
although  of  diminishing  importance.  In  no  sense, 
therefore,  can  they  be  called  reversions. 

C.  ACQUIRED   CHARACTERS   RESEMBLING  ANCESTRAL   ONES 

Sometimes  the  drunken  descendant  of  a  drunken 
great-grandparent  has  acquired  this  characteristic 
through  his  own  initiative  quite  aside  from  any  ances- 
tral contribution  to  his  germplasm.  This  is  not  rever- 
sion, but  reacquisition  resembling  the  ancestral  con- 
dition. 

Again,  tame  animals  that  run  wild  acquire  habits 
resembling  those  of  their  wild  ancestors,  but  this  is 
not  necessarily  reversion.  It  is  the  natural  response 
of  feral  animals  to  the  conditions  of  wild  life. 

D.    CONVERGENT    VARIATION 

The  European  hedgehog,  Erinaceus,  an  insectivore, 
the  American  porcupine,  Erithizon,  a  rodent,  and  the 
Australian  spiny  anteater,  Echidna,  a  monotreme,  are 
all  mammals  which  have  developed  in  a  similar  manner 


OLD  TYPES  AND  NEW  199 

the  very  peculiar  device  of  dermal  spines.  There  is  no 
reason,  however,  for  regarding  this  character  as  due 
to  descent  from  a  common  spiny  ancestor.  It  is  not 
reversion  to  an  ancestral  type,  but  rather  a  case  of ^ 
convergent  variation.  Similarity  does  not  always  indi- 
cate genetic  continuity. 

In  the  case  of  birds  albinism,  melanism  and  flavism 
are  modifications  of  ordinary  pigmentation  which 
appear  irregularly  among  many  different  species  as 
pathological  "sports,"  but  no  one  of  these  conditions 
can  be  regarded  as  reversions  to  ancestral  white,  black, 
or  yellow  types. 

E.    REGRESSION 

Galton's  "law  of  regression"  refers  to  the  widespread 
phenomenon  already  explained  of  a  constant  swinging 
back  to  mediocrity  which  the  breeder  must  oppose  with 
continual  selection  in  order  to  maintain  the  standard 
of  any  particular  strain.  We  have  seen  that  within 
a  "pure  line,"  regression  is  complete  and  that  in  popu- 
lations made  up  of  a  mixture  of  pure  lines  it  is  a  fac- 
tor that  must  invariably  be  considered.  Regression, 
however,  has  to  do  with  fluctuating  variations  and  does 
not  bring  about  a  permanent  change  of  type.  It 
should,  therefore,  not  be  confused  with  reversion. 

3.  EXPLANATION  OF  REVERSION 

Darwin,  who  did  not  always  differentiate  between 
reversion  and  atavism,  suggested  that  reversion  was 
due  sometimes  to  the  action  of  a  more  natural  en- 


200  GENETICS 

vironment,  as  in  the  case  of  animals  set  free  after  hav- 
ing been  in  captivity,  and  sometimes  to  hybridization, 
since  there  seems  to  be  a  general  tendency  of  hybridized 
organisms  to  "revert"  to  ancestral  types. 

It   is   now   known   that    reversion,   like   atavism,   is/ 
simply  a  case  of  latent  characters  becoming  apparent'^ 
according  to  the  Mendelian  principle  of  segregation,  i 
To  quote  Davenport:    "There  is  nothing  more  mys- 
terious about  reversion,  from  the  modern  standpoint, 
than  about  forming  a  word  from  the  proper  combina- 
tion of  letters." 

4.  THE  ART  AND  SCIENCE  or  BREEDING 

The  art  of  breeding  animals  and  plants  has  been 
practiced  from  very  early  times  while  the  science  of 
maintaining  old  types  and  initiating  new  ones  is  of 
comparatively  recent  origin. 

Some  of  the  methods  that  have  been  employed  with 
varying  degrees  of  success  are: — 

A.  Mass  selection; 

B.  Pedigree  breeding ; 

C.  Inbreeding; 

D.  Hybridization; 

E.  Genotypic  selection. 

A.    MASS    SELECTION 

The  natural  thing  to  do  in  the  maintenance  or  im- 
provement of  cultivated  plants  and  domestic  animals  is 
to  select  seeds  from  the  best  looking  plants  and  to 


OLD  TYPES  AND  NEW  201 

breed  together  from  the  flock  or  herd  those  animals 
which    appear    most    desirable.      This    has    been    the 
method  from  the  beginning  and  there  is  a  reason  for 
the  considerable  degree  of  success   that  has   followed 
this  obvious  mode  of  procedure.    The  method,  however  ,i 
has  its  limitations  because  it  is  entirely  phenotypic  andS 
the  breeder  is  sure  to  find  with  Dryden  that  "all  as  I 
they  say  that  glitters  is  not  gold." 

Two  methods  of  mass  selection,  as  applied  to  plants, 
may  be  mentioned  that  differ  in  the  extent  to  which 
the  environment  is  recognized  as  a  contributing  factor. 

a.  The  Method  of  Hallet 

The  English  wheat-grower  Hallet  formulated  this 
method  in  1869  and  it  has  been  in  common  use  for  a 
long  time.  It  consists  in  placing  the  organisms  to  be 
bred  in  the  very  best  possible  environment  and  then 
choosing  those  individuals  making  the  best  showing 
as  the  stock  from  which  to  breed  further,  a  procedure 
based  upon  the  deep-seated  belief  that  acquired  char- 
acters are  inherited. 

For  example,  in  a  field  of  wheat,  plants  near  the 
edge  of  the  field  which,  from  lack  of  crowding  or  by 
reason  of  proximity  to  an  extra  local  supply  of  fer- 
tilizer or  any  other  favorable  environmental  factor, 
make  a  more  vigorous  growth  than  their  neighbors, 
are  selected  in  the  hope  that  the  gains  made  by  them 
will  be  maintained  in  their  offspring. 

We  have  seen  that  it  is  very  questionable  whether 
acquired  characters  due  to  environmental  conditions 


202  GENETICS 

play  any  role  whatever  in  heredity.  The  phenotypic 
character  does  not  always  indicate  what  the  germ- 
plasm  will  subsequently  do,  and  when  the  true  geno- 
typic  constitution  of  the  germplasm  is  still  further 
masked  by  the  temporary  fluctuations  caused  by  a 
modified  environment  it  is  increasingly  difficult  to  select 
wisely  from  the  display  of  variants  those  which  will 
produce  the  best  ancestors  for  the  future  stock. 

That  this  common  procedure  of  selecting  the  best- 
appearing  animal  in  the  flock  and  the  biggest  ear  of 
corn  in  the  bin  has  met  with  a  large  degree  of  success 
in  the  past  is  due  entirely  to  the  fact  that  in  many 
instances  the  phenotypic  character  is  an  actual  express 
sion  of  the  genotypic  constitution.    This  is  not  always 
the  case,  however,  and  we  cannot  now  fail  to  see  thali 
the  method  is  blind  and  full  of  error.     Its  successes\ 
are  due  to  the  indirect  results  of  chance  rather  than  j 
to  a  direct  control  of  the  factors  of  heredity. 

b.  The  Method  of  Rimpau 

Contrasted  with  the  method  of  Hallet  of  augment- 
ing acquired  characters  and  then  selecting  from  them 
the  best  display,  is  the  method  of  Rimpau,  who  ex- 
perimented for  two  decades  with  various  grains  and, 
finally,  among  other  results,  produced  the  famous 
Schlandstedt  barley. 

Rimpau's  method  is  to  sow  grain  under  ordinary 
conditions  with  a  minimum  rather  than  a  maximum 
amount  of  fertilizer  and  then  to  select  individuals, 
neither  from  the  rich  spots  nor  from  the  edges  of  the 


OLD  TYPES  AND  NEW  203 

field  where  there  is  little  crowding,  but  from  situations 
where  the  environmental  conditions  are  ordinary  or 
even  unfavorable.  Individuals  making  a  good  showing 
under  such  usual,  or  even  adverse,  conditions  are  wor- 
thy by  nature  rather  than  by  nurture  and  are  conse- 
quently most  desirable  as  progenitors  of  future  stock. 
By  this  method  the  attempt  is  not  to  keep  the  progeny 
of  single  individuals  separate,  but  to  mass  together 
the  best  as  they  appear  under  ordinary  normal  environ- 
ment. 

This    again    is    an    indirect    method    of    procedure, 
although    the    character    of    the    germplasm    is    more 
nearly  hit  upon  in  this  way  than  by  Hallet's  method, 
since  the  mask  of  temporary  accessory  modifications/ 
is  stripped  so  far  as  possible  from  the  somatoplasm,  \ 
and   the  phenotype  made   to   approximate   the   geno-/ 
typical  constitution. 

B.   PEDIGREE   BREEDING 

Mass  selection,  or  the  choosing  of  a  number  of  indi- 
viduals out  of  a  population  to  be  the  progenitors  of 
the  next  generation,  is  subject  to  repeated  backsliding 
to  mediocrity  and  consequently  the  selection  must  be 
made  over  and  over  again  in  each  generation.  A 
greater  degree  of  success  than  is  possible  by  this 
method  has  followed  attempts  to  isolate  single  self- 
fertilizing  individuals  that  manifest  the  desired  quali- 
ties and  to  establish  pedigrees  from  this  isolated  stock. 
This  is  Johannsen's  method  of  the  pure  line  and  is 
particularly  applicable  to  self-fertilizing  plants,  al- 


204  GENETICS 

though  it  may  be  extended  to  clones,  parthenogenetic 
lines  and  to  homozygous  crosses. 

A  quotation  from  the  memoirs  of  the  Manchu  em- 
peror K'ang-Hsi,  1662-1723,  translated  from  VEm- 
pire  Chinois,  E.  R.  Hue,  will  illustrate  an  early 
application  of  the  pedigree  method.  "On  the  first  day 
of  the  sixth  moon  I  was  walking  in  some  fields  where 
rice  had  been  sown  to  be  ready  for  the  harvest  in  the 
ninth  moon.  I  observed  by  chance  a  stalk  of  rice 
already  in  ear.  It  was  higher  than  all  the  rest  and 
ripe  enough  to  be  gathered.  I  ordered  it  brought  to 
me.  The  grain  was  very  fine  and  well  grown,  which 
gave  me  the  idea  to  keep  it  for  a  trial  and  see  if  the 
following  year  it  would  preserve  its  precocity.  It 
did  so.  All  the  stalks  which  came  from  it  showed  ear 
before  the  usual  time  and  were  ripe  in  the  sixth  moon. 
Each  year  has  multiplied  the  produce  of  the  preceding, 
and  for  thirty  years  it  is  the  rice  which  has  been 
served  at  my  table.  It  is  the  only  sort  which  can  ripen 
north  of  the  great  wall,  where  the  winter  ends  late  and 
begins  very  early ;  but  in  the  southern  provinces,  where 
the  climate  is  milder  and  the  land  more  fertile,  two 
harvests  a  year  may  be  easily  obtained,  and  it  is  for 
me  a  sweet  rejection  to  have  procured  this  advantage 
for  my  people" 

In  the  last  century  the  isolation  of  pure  lines  was 
practiced  notably  by  the  Englishman  LeCoutour,  who 
isolated  150  varieties  of  wheat  and  by  the  Scotchman 
Shirreff,  who  worked  with  various  cereals. 

In  recent  years  the  principle  has  been  extensively 
applied  with  remarkable  results,  particularly  by 


OLD  TYPES  AND  NEW  205 

Nilsson  of  Svalof  in  Sweden,  upon  peas,  potatoes, 
clovers,  grasses  and  grains. 

Among  others  in  America,  Hays  has  isolated  pedi- 
grees of  wheat  at  the  Minnesota  Agricultural  Experi- 
ment Station,  which  within  a  decade  have  been  grown 
on  thousands  of  acres  and  have  "made  possible  the 
increased  production  of  wheat  throughout  the  northern 
States  and  Canada." 

An  isolation  method  that  has  been  successfully  ap- 
plied to  the  sugar-beet  industry  is  that  of  Vilmorin. 
The  seeds  from  each  plant  to  be  tested  are  sown  in 
separate  beds  from  which  upon  maturity  samples  are 
taken  and  tested  for  sugar  content.  The  plants  from 
the  bed  furnishing  the  sample  containing  the  highest 
percentage  of  sugar  are  then  used  as  the  seed  pro- 
ducers for  the  next  generation.  In  this  way  by  con- 
tinual selection  an  improved  strain  is  maintained. 

C.    INBREEDING 

When  breeding  is  kept  up  between  individuals  of 
the  same  stock  it  tends  to  perpetuate  or  preserve  the 
distinctive  characteristics  of  that  stock,  a  practice 
that  was  advocated  in  the  Mosaic  law, — "Thou  shalt 
not  let  thy  cattle  gender  with  a  diverse  kind;  thou 
shalt  not  sow  thy  field  with  mingled  seed."  (Levit. 
XIX  :19.) 

Numerous  experiments  to  test  the  effect  of  inbreed- 
ing have  been  carried  out  upon  various  organisms. 

Darwin,  for  instance,  planted  morning-glories, 
derived  from  the  same  stock  of  seeds,  in  two 


206  GENETICS 

beds  which  were  laid  out  side  by  side,  that  is,  in  an 
environment  as  nearly  the  same  as  possible,  but  with 
half  of  the  beds  screened  from  insects  which  usually 
transfer  pollen  from  flower  to  flower.  In  the  screened 
half  where  all  insects  were  excluded  the  flowers  were 
of  necessity  self -fertilized,  while  in  the  exposed  half 
they  were  presumably  cross-pollinated  by  insects  which 
had  free  access  to  them.  The  seeds  produced  in  the 
two  beds  were  kept  separate  and  the  experiment  was 
continued  for  ten  years,  so  that  at  the  end  of  that  time 
two  lots  of  morning-glories,  one  self-fertilized  for  ten 
generations  and  the  other  presumably  cross-pollinated 
for  the  same  length  of  time,  were  obtained  for  com- 
parison. The  criterion  Darwin  used  was  the  vigor  of 
the  plants  as  shown  by  the  length  of  the  vine.  He 
found  that  the  cross-pollinated  plants  were  to  the  self- 
pollinated  ones  as  100  to  53,  and  his  conclusion  was, 
consequently,  that  cross-pollination  is  beneficial  and 
self-pollination  is  detrimental. 

Ritzema-Bos  inbred  rats  for  twenty  generations. 
For  the  first  ten  generations  the  average  number  of 
young  per  litter  was  7.5,  while  for  the  last  ten  genera- 
tions it  fell  to  3.2. 

Weismann  inbred  mice  for  twenty-nine  generations 
and  obtained  a  parallel  result.  For  the  first  ten  gen- 
erations the  average  number  per  litter  was  6.1,  for  the 
second  ten  generations  5.6,  and  for  the  last  nine  gen- 
erations 4.2. 

Dr.  Helen  King,  on  the  other  hand,  practiced  close 
inbreeding  with  white  rats  for  40  generations  com- 
prising over  20,000  individuals  obtained  by  mating 


OLD  TYPES  AND  NEW  207 

brothers  and  sisters  from  the  same  litter  at  the  end  of 
which  time  the  animals  were  larger  and  more  vigorous 
than  those  not  inbred. 

Shull  found  in  growing  Indian  corn  that  loss  of 
vigor  results  from  continual  self-fertilization,  and 
many  breeders  have  had  similar  experiences  with  ani- 
mals as  well  as,  other  plants. 

In  the  case  of  the  pomace  fly,  Drosophila,  Castle 
inbred  brother  and  sister  for  fifty-nine  generations 
without  diminishing  the  fertility  of  the  line. 

Hornaday  cites  the  case  of  the  deer  in  the  royal 
herd  at  Windsor  which  arose  from  one  male  and  two 
females  introduced  from  New  Zealand  in  1862.  The 
herd  now  numbers  20,000  and  shows  no  signs  of  de- 
terioration. 

No  arbitrary  law  with  respect  to  the  effects  of  in- 
breeding upon  vigor  and  fertility  can  be  laid  down, 
therefore,  which  will  apply  equally  to  all  cases. 

Nature  has  secured,  often  by  elaborate  devices,  a 
separation  of  the  sexes,  especially  among  the  higher 
organisms,  and  in  consequence  there  has  arisen  an 
unavoidable  necessity  of  outcrossing.  The  intricate 
adaptations  existing  between  insects  and  flowers,  for 
example,  seem  to  be  directed  entirely  toward  insuring 
outcrossing  among  plants. 

There  are,  on  the  other  hand,  various  well  known 
provisions  in  nature  to  insure  inbreeding.  The  ma- 
jority of  plants  are  probably  self-fertilized  while  her- 
maphroditic animals,  which  sometimes  at  least  are  self- 
fertilized  particularly  among  the  lower  forms,  are  very 
common. 


208  GENETICS 

The  whole  matter  of  inbreeding  and  the  part  it  plays 
in  emphasizing  defects  has  received  a  fresh  interpre- 
tation in  the  light  of  Mendelism. 

There  is  a  widespread  popular  belief  that  inbreeding 
is  injurious  and  that  it  is  necessary  to  outcross  in 
order  to  maintain  the  vigor  and  avoid  the  defects  of 
any  line,  but  inbreeding  in  itself  may  not  necessarily 
be  injurious.  The  consequence  of  inbreeding  as  shown 
by  the  working  of  Mendelian  laws  is  that  latent  or 
recessive  characters  tend  to  become  homozygous  and 
so  brought  to  the  surface,  while  outcrossing  brings 
about  the  formation  of  heterozygous  traits  which  mask 
recessive  characters  and  render  them  ineffective. 

In  the  case  of  mankind,  consanguineous  marriage 
of  various  degrees  has  long  been  forbidden  by  law  or 
custom  in  many  races,  particularly  among  the  Jews, 
Mohammedans,  Indians  and  Romans.  On  the  other 
hand,  the  Persians,  Greeks,  Phoenicians  and  Arabs  have 
freely  practised  inbreeding,  while  one  of  the  longest 
of  known  human  pedigrees,  a  royal  line  of  Egypt,  is 
notorious  for  close  inbreeding,  even  to  the  mating  of 
brother  and  sister. 

There  has  been  a  greater  degree  of  inbreeding  in 
the  Puritan  stock  of  New  England  than  is  commonly 
realized.  David  Starr  Jordan  points  out  that  a  child 
of  to-day,  supposing  no  inbreeding  of  relatives  had 
occurred,  would  have  had  in  the  time  of  William  the 
Conqueror,  thirty  generations  ago,  8,598,094,592 
living  ancestors.  If  this  theoretical  supposition  were 
really  so,  it  would  seem  quite  possible  for  every  New 
Englander  to-day  to  have  at  least  one  ancestral 
representative  who  won  glory  under  William. 


OLD  TYPES  AND  NEW  209 

The  difference  between  the  unthinkable  number 
given  above  and  the  actual  number  of  probable  an- 
cestors alive  thirty  generations/, :ago  emphasizes  the 
fact  that  inbreeding  must  have  occurred  freely. 

Cousin-marriages,  although  producing  a  high  per- 
centage of  defects,  do  not  necessarily  produce  unde- 
sirable traits.  They  simply  bring ,  out  latent  or  re- 
cessive characters  for  the  reason  that  under  these  con- 
ditions defect  meets  defect  instead  of  the  opposite 
normal  condition  which  would  dominate  the  defect  and 
cause  it  not  to  appear. 

Since  a  recessive  trait  may  be  properly  regarded  as 
the  absence  of  a  positive  dominant  character,  it  more 
frequently  stands  for  an  undesirable  feature  than 
otherwise.  Thus  it  comes  about  that  inbreeding,  by 
combining  negative  features,  may  "produce"  a  defec- 
tive strain. 

Outcrossing  always  increases  heterozygous  combi- 
nations in  the  germplasm  and  covers  up  undesirable^ 
recessive  traits  through  the  introduction  of  additional  \ 
dominant  traits.  Inbreeding,  on  the  contrary,  tends  ; 
to  simplify  the  germplasm,  that  is,  to  make  it  more  ; 
homozygous,  and  so  to  bring  recessive  defects  to  the  ; 
surface. 

D.    HYBRIDIZATION  y/ 

Among  the  first  to  use  the  powerful  tool  of  hybridi- 
zation were  Koelreuter,  1733-1806,  in  Germany,  and 
Knight,  1758-1838,  in  England.  These  pioneer  trans- 
gressors of  the  Mosaic  law  cited  in  the  foregoing  para- 
graph, opened  up  a  broad  road  to  the  army  of  the 
Mendelians  who  were  to  follow  them.  Not  only  have 


210  GENETICS 

individuals  of  two  varieties  showing  hereditary  differ- 
ences been  hybridized  but  successful  crosses  have  been 
artificially  brought  about  between  individuals  belong- 
ing to  different  species,  to  different  genera  and  even 
to  different  groups  still  more  distantly  related  to  each 
other. 

It  may  be  possible  to  point  out  at  least  two  general 
methods  of  utilizing  hybridization. 

a.  The  Method  of  Burbank 

This  is  a  method  of  greatly  increasing  the  number 
of  variants  by  promiscuous  hybridization  and  then  of 
eliminating  all  except  those  of  a  desired  phenotypic 
combination.  Indirectly  it  depends  upon  the  principle 
of  the  segregation  of  unit  characters  which  makes 
possible  rearrangements  of  these  characters  according 
to  the  laws  of  chance.  The  characters  themselves  re- 
main unchanged,  since  nothing  new  is  produced  by 
hybridization  except  new  arrangements  of  existing 
characters. 

The  spectacular  success  of  Luther  Burbank  in 
"creating"  new  plant  forms  is  due  largely  to  his  very 
extensive  hybridizations,  his  skill  in  detecting  among 
the  varying  progeny  the  winning  phenotype  and  his 
ruthless  elimination  of  the  great  majority  of  variations 
that  do  not  quite  fill  his  requirement. 

The  successful  combinations  mustjDe  propagated  in 
most  instances  asexually  by  grafting,  cuttings,  bulbs, 
etc.,  rather  than  sexually  through  the  medium  of 
seeds,  because  new  genotypes  which  will  breed  true  are 


OLD  TYPES  AND  NEW 

not  necessarilyj^oLated  by  this^  procedure.  The  conse- 
quence is  that  Burbank's  method  cannot  be  utilized 
in  animal  breeding  to  any  great  extent  where  the  main- 
tenance of  a  desirable  strain  by  asexual  propagation 
is  out  of  the  question. 

It  will  be  seen  that  this  method  is  fortuitous  and  to 
a  certain  extent  unscientific  in  that  no  one  can  repeat 
the  exact  conditions  of  the  experiment  and  arrive  at 
the  same  results.  It  depends  upon  the  chance  mixing 
up  of  a  large  number  of  possibilities  and  then  in  not 
being  distracted  or  blinded  by  the  good  while  selecting 
the  best.  In  the  hands  of  a  skilful  plant  breeder 
with  unlimited  resources  at  his  command  it  may 
result  in  much  practical  achievement,  but  it  does 
not  particularly  illuminate  the  path  of  other  breeders 
who  wish  to  repeat  the  experiment.  It  is  after  all 
selection  of  phenotypes  and,  therefore,  forever  open  to 
error,  since  phenotypes  do  not  always  indicate  what 
the  behavior  of  their  constituent  genotypes  will  be  in 
heredity. 

b.  The  Method  of  Mendel 

The  method  of  Mendel,  like  the  foregoing,  depends 
upon  hybridization  with  the  difference  that  the  desired 
combination  is  sought  directly  by  definite  predetermined 
crosses,  according  to  the  expectations  of  the  Mendelian 
ratios,  rather  than  through  the  random  result  of  for- 
tuitous combinations.  It  is  a  method  which  has  been 
rendered  possible  by  the  determination  of  Mendel's  laws 
of  dominance,  and  of  the  independence  and  segregation 
of  unit  characters  which  give  to  the  experimental 


GENETICS 

breeder  definite  expectations  and  a  method  of  procedure. 

If,  upon  hybridization,  the  desired  character  be- - 
haves  like  a  recessive,  then  all  that  is  necessary  to\ 
establish  a  pure  stock  exhibiting  the  character  in  ques-  ( 
tion,  is  to  breed  two  recessives  together,  because  reces-  J 
sives  are  always  homozygous  and,  regardless  of  their  / 
ancestry,  breed  true. 

On  the  other  hand,  if  the  desired  character  proves  , 
to  be  a  dominant,  then  it  is  necessary  to  determine 
whether  it  is  present  in  a  duplex  or  a  simplex  condition ;  j 
in  other  words,  whether  it  is  homozygous  or  hetero- 
zygous,  for  only  homozygous   organisms   breed   true. 
Establishing  a  strain  consists,  consequently,  in  making 
an   organism   homozygous. 

The  test  to  determine  whether  a  dominant  character 
is  homozygous  or  heterozygous,  that  is,  whether  it 
will  breed  true  or  not,  can  be  made  by  a  single  cross 
according  to  the  procedure  outlined  in  paragraph  8 
of  Chapter  V.  If,  upon  crossing  the  individual  to  be 
tested  with  a  recessive,  it  produces  an  entirely  dominant 
progeny,  then  its  germplasm  is  duplex  for  this  charac- 
ter, and  it  will  always  reproduce  the  character  in 
either  duplex  or  simplex  condition  according  to  what- 
ever cross  may  be  made  with  it.  When  crossed,  for 
instance,  with  another  duplex  dominant  like  itself,  a 
pure  homozygous  strain  of  the  character  in  question 
will  be  perpetuated. 

If,  on  the  contrary,  the  dominant  character  to  be 
tested  proves  to  be  simplex  or  heterozygous,  as  de- 
termined by  the  fact  that,  when  crossed  with  a  re- 
cessive, 50  per  cent  of  the  progeny  are  recessive,  then 


OLD  TYPES  AND  NEW 

it  requires  more  than  a  single  generation  to  establish 
a  homozygous  dominant  strain. 

In  random  inbreeding  of  diverse  strains  if  the  re- 

A 

(cessives  are  constantly  eliminated  as  they  appear,  a 
/population  is   gradually   obtained  which  is   composed 
\  of  an  increasing  number  of  dominants  so  that  after 
only  a  few  generations  the  chances  are  much  reduced 
that  recessives  will  again  appear,  which  means  the  prac- 
tical purity  of  the  strain. 

E.    GENOTYPIC     SELECTION 

/"  The  success,  however,  of  any  method  of  originating 
\  new  types  of  organisms  or  of  improving  old  ones  must 
/  depend  in  the  long  run  upon  the  selection  of  germinal 

differences. 

j  The  difficulty  here  of  course  lies  in  the  fact,  that  we 
may  only  know  the  potential  germplasm  from  its  per- 
formance in  producing  somatoplasm,  but  Mendelism 
with  its  analysis  of  the  genes  through  breeding^  cer- 
tainly has  gone  a  long  way  toward  making  genotypic 
selection  possible  and  definite.  Moreover,  the  preserva- 
tion and  exploitation  of  mutations  when  they  are  known 
is  certainly  along  the  line  of  genotypic  selection,  since 
mutations  when  isolated  may  become  the  progenitors  of 
desirable  new  lines.  Accordingly  until  the  secret  of 
the  origin  of  mutations  is  solved  the  work  of  the  suc- 
cessful breeder  consists  to  a  very  large  extent  in  simply 
taking  what  mutations  nature  spontaneously  furnishes 
to  him  rather  than  in  attempting  to  force  nature  into 
producing  something  new. 


214  GENETICS 

5.  HETEROSIS 

When  hybrids  are  formed  the  first  hybrid  generation  / 
not  only  shows  more  variability  but  also  more  vigor  \ 
than  the  parental  strains  and  this  vigor  is  in  proportion  k 
to  the  number  of  factors  in  which  the  parents  differ,  be- 
cause in  hybridization  there  is  a  pooling  of  hereditary 
resources.     Such  hybrid  vigor  is  termed  hetergsis. 

East  and  Hayes  describe,  for  example,  a  cross  be- 
tween two  different  wild  varieties  of  tobacco  in  which 
the  average  height  of  over  fifty  plants  of  each  of  the 
two  wild  parents  was  31  and  54*  inches  respectively. 
Of  an  equal  number  of  hybrids  of  the  first  generation 
the  average  height  was  over  67  inches  under  the  same 
environmental  conditions.  Shull  and  East,  working 
separately  upon  maize,  came  to  the  same  conclusion, 
namely,  that  the  first  hybrid  generation  following  an 
artificial  cross  is  decidedly  more  vigorous  than  the 
parental  stocks  from  which  it  is  derived.  This  is  shown 
in  Figures  45  and  46. 

The  mule  is  a  notorious  hybrid  that  possesses  more 
"kick"  than  its  parents. 


FIGS.  45  and  46. — Results  of  crossing  two  inbred  strains  of  corn. 
At  the  left  in  Fig.  45  are  two  inbred  varieties.  The  tall  corn 
at  the  right  is  the  result  of  crossing  them.  In  Fig.  46,  the 
basket  at  the  right  represents  the  average  production  of  two 
inbred  strains  after  three  generations  of  inbreeding — 61 
bushels  per  acre.  The  basket  at  the  left  shows  the  first  gen- 
eration results  from  crossing  them — 101  bushels  per  acre. 
After  East  and  Hayes. 


CHAPTER  X 

THE  CARRIERS  OF  THE  HERITAGE 

1.  INTRODUCTION 

HEREDITY,  as  has  been  shown  in  the  introductory 
chapter,  is  essentially  a  matter  of  continuity  between 
\  succeeding  generations  of  living  organisms.  This  con- 
tinuity may  be  direct,  as  when  a  mother  protozoan 
divides  into  two  daughters,  or  it  may  be  indirect,  as 
illustrated  by  the  relationship  of  a  father  and  son, 
an  uncle  and  nephew,  or  any  other  relatives  of  varying 
degrees  of  kinship  which,  taken  singly  or  collectively, 
are  somatoplasms  derived  from  a  common  stream  of 
gern.plasm. 

It  is  the  purpose  of  the  present  chapter  to  consider 
this  material  continuity  between  succeeding  genera- 
tions and  to  discover,  if  possible,  just  what  are  the 
carriers  of  the  heritage  from  one  generation  to  another. 
To  this  end  it  will  be  necessary  in  the  first  place  to 
take  up  what  is  meant  by  the  "cell  theory." 

2.  THE  CELL  THEORY 

In  1838^839  the  "cell  theory"  of  Schleiden  and 
Schwann,  which  affirms  that  all  organisms,  both  plant 
and  animal,  are  made  up  of  cellular  units,  had  its  birth. 

215 


216  GENETICS 

Robert  Hooke,  as  early  as  1665,  had  described 
"little  boxes  or  cells  distinguished  from  one  another" 
which  he  saw  in  thin  slices  of  cork,  and  to  him  is  due 
the  rather  unfortunate  use  of  the  term  "cell"  which 
has  survived  in  biological  writings  to  this  day.  The 
reason  this  term  is  unfortunate  is  because  walls,  which 
are  ordinarily  the  characteristic  feature  of  any  cell, 
such  as  a  prison  cell,  are  usually  the  least  important 
part  of  the  structure  of  a  living  cell,  often  indeed 
being  entirely  absent. 

3.  A  TYPICAL  CELI, 

A  typical  undifferentiated  cell  is  represented  dia- 
grammatically  in  Figure  47.  Near  the  center  of  the 


Cell  wall 
Cytoplasm 
Centrosome 
Nuclear  membrane 

Nucleus 
Chromatin  network 


FIG.  47. — Diagram  of  a  typical  cell. 

cell  the  nucleus  is  shown  surrounded  by  a  nuclear  mem- 
brane. The  nucleus,  in  common  with  the  enveloping 
cytoplasm,  is  made  up  of  living  substance  called  proto- 
plasm (Hugo  von  Mohl,  1846),  and  around  the  whole 
there  is  usually  formed  a  wall  or  membrane  which 


THE  CARRIERS  OF  THE  HERITAGE 

serves  to  separate  one  cell  from  another.  Within  the 
protoplasm  there  may  be  a  considerable  amount  of  non- 
living substance  in  the  form  of  saltsL  pigments,  oil- 
drops,  water,  and  other  inclusions  of  various  kinds. 

The  nucleus  is  to  be  regarded  as  the  headquarters 
of  the  whole  cell,  since  changes  which  the  cell  under- 
goes seem  to  be  initiated  in  it,  while  cells  deprived  of 
their  nuclei  cannot  long  survive.  A  single  instance  will 
serve  to  show  the  vital  part  which  the  nucleus  plays 
in  the  life-history  of  the  cell.  In  1883,  Gruber  found 
that  after  rocking  a  thin  cover-glass  back  and  forth  in 
a  drop  of  water  containing  a  collection  of  the  proto- 
zoan Stentor,  which  has  a  long  chain-like  nucleus, 
these  tiny  animals  could  thus  be  cut  into  fragments, 
which  would  in  some  instances  recover  from  the  opera- 
tion and  regenerate  into  complete  individuals.  Only 
those  pieces,  however,  which  contained  a  fragment  of 
the  nucleus  regenerated  into  new  Stentors,  while  pieces 
of  relatively  large  size  which  lacked  a  fragment  of 
nuclear  substance  very  soon  disintegrated. 

The  nucleus,  it  should  be  said,  is  made  up  of  more 
than  one  substance,  a  fact  that  is  easily  demonstrated 
by  processes  of  staining,  in  which  certain  dyes,  through 
chemical  union,  stain  a  part  but  not  the  whole  of  the 
nuclear  substance.  The  part  most  easily  stained  is 
called  chromatin,  that  is  "colored  material,"  and  during 
certain  phases  of  cell  life  the  chromatin  masses  to- 
gether within  the  nucleus  into  visibly  definite  structures 
or  bodies  termed  chromosomes. 

Throughout  all  the  various  cells  that  make  up  the 
individuals  of  any  one  species  these  chromosomes  ap- 


218  GENETICS 

pear  to  be  practically  constant  in  number  with  some 
exceptions  to  be  mentioned  later  in  connection  with  sex. 
This  law  of  the  constant  chomosome  number  for  any 
species  was  first  stated  by  Boveri  in  1900. 

The  chromosomes  of  different  organisms  vary  in 
number  from  two  in  the  worm  Ascaris  up  to  perhaps 
1600,  according  to  Haecker  ('09),  in  certain  radiolaria. 
A  recent  list  records  the  number  of  chromosomes  typical 
for  960  different  animals.1  Species  which  apparently 
are  closely  related  may  differ  widely  with  respect  to  the 
number  of  their  chromosomes,  while  species  of  unques- 
tionably remote  relationship  may  have  an  identical 
number  of  chromosomes  in  each  of  their  cells.  The 
number  of  chromosomes  characteristic  for  a  species, 
therefore,  is  in  no  way  an  index  to  the  complexity  or 
degree  of  differentiation  of  the  species. 

Besides  the  nucleus  there  may  often  be  identified  in 
the  cytoplasm  of  the  animal  cell  a  tiny  body  known  as 
the  centrosome.  At  certain  times  in  the  life-cycle  of 
a  cell  the  centrosome  becomes  the  focal  point  of  pecul- 
iar radiating  lines,  which  play  an  important  part  in 
the  behavior  of  the  cell,  particularly  during  the  period 
of  division. 

Every  cell  passes  through  a  cycle  of  life  which  may 
be  compared  with  that  common  to  individuals.  It  is 
born  from  another  cell;  passes  through  a  vigorous 
youth  characterized  by  growth  and  transformation; 
attains  maturity  when  the  metamorphoses  of  its  earlier 
life  give  place  to  a  considerable  degree  of  stability ;  and 
finally,  after  a  more  or  less  extended  period  of  normal 
Vowr.  of  Morphology,  vol.  34,  pp.  1-67,  1920. 


THE  CARRIERS  OF  THE  HERITAGE 

activity  reaches  old  age,  and  death  completes  the 
cycle.  In  most  instances,  however,  before  this  final 
phase  is  reached,  the  cell  gives  place  to  daughter- 
cells  through  fission,  after  the  manner  of  most  proto- 
zoans, and  a  new  cell  cycle  is  begun. 

Sometimes  the  road  of  differentiation  has  been  trav- 
eled so  far  that  it  is  apparently  impossible,  as  in  the 
case  of  the  complicated  brain-cells,  to  retrace  these 
steps  of  differentiation  and  begin  again.  In  such  in- 
stances the  outfit  of  cells  provided  in  the  embryo 
determines  the  numerical  limit  of  the  cells  available 
throughout  life.  When  this  supply  is  exhausted  no 
more  cells  appear  to  replace  those  which  have  been 
worn  out. 

4.  MITOSIS 

The  ordinary  process  by  which  two  cells  are  made 
out  of  one  is  termed  mitosis.  It  occurs  constantly, 
and  particularly  during  growth,  in  all  cellular  organ- 
isms. A  series  of  diagrams,  modified  from  Boveri,  illus- 
trating the  typical  phases  of  mitosis  is  given  in  Figures 
48  to  57. 

The  restmg  cell  (Fig.  48)  is  characterized  by  the 
presence  of  a  nuclear  membrane,  a  single  centrosome, 
and  by  a  chromatin  network  within  the  nucleus.  In  the 
beginning  of  the  pro  phase  (Fig.  49)  the  centrosome 
has  divided  into  two  parts,  while  in  the  early  prophase 
(Fig.  50)  the  two  centrosomes  have  moved  farther 
apart  and  definite  separate  chromosomes  have  formed 
out  of  the  chromatin  network.  The  prophase^  proper 
(Fig.  51)  is  marked  by  the  vanishing  of  the  nuclear 


220 


GENETICS 


membrane  and  the  more  compact  form  of  the  chromo- 
somes. At  the  end  of  the  prophase  (Fig  52)  the  chro- 
mosomes have  come  to  lie  at  the  equator  of  the  cell, 


fig.  48.  The  Resting  Cell  Kg.  49.  Beginning  Propncae  Flg.60.  Early  Prophets 


F1g»  61.  Prophase        Fig.  52.  End  of  Prophase         Fig.  63.  Mataphate 


Fig.  64.  Beginning  Anaphase 


Fig.  66.  Anaphase 


Fig.  66.  Beginning  Telophase  Kg.  67.  End  of  Telophaw 

Pioi.  48-57. — Diagrams  illustrating  mitosis.     After  Boveri. 

being  connected  by  the  mantle  fibers  with  the  centro- 
somes,  each  of  which  now  occupies  a  polar  position. 
In  the  metaphase  (Fig.  53)  the  chromosomes  split 
lengthwise,  and  at  the  beginning  of  the  anaphase  (Fig, 


THE  CARRIERS  OF  THE  HERITAGE 

54)  these  half-chromosomes  commence  to  separate  from 
each  other  and  to  move  toward  the  poles,  while  the 
mantle  fibers  shorten.  During  the  anaphase  (Fig.  55) 
the  cell  body  lengthens  and  begins  to  divide,  while  the 
migration  of  the  half-chromosomes  toward  the  poles  is 
completed.  In  the  begirmmg  of  the  telophase  (Fig. 
56)  the  half-chromosomes  grow  until  they  attain  full 
size  and  the  division  of  the  cell  body  into  two  parts 
becomes  complete.  The  mantle  fibers  have  disappeared 
and  the  nuclear  membrane  begins  to  reform  around 
the  chromosomes.  Finally,  at  the  end  of  the  telophase 
(Fig.  57)  the  nuclear  membrane  becomes  complete, 
the  chromosomes  break  up  into  a  chromatin  network, 
and  two  resting  cells  take  the  place  of  the  single  one 
with  which  the  process  began  (Fig.  48). 

5.  SEXUAL  REPRODUCTION 

The  mechanism  by  means  of  which  two  cells  unite 
to  make  one  in  sexual  reproduction  is  quite  as  com- 
plicated as  that  of  mitosis  by  which  one  cell  is  trans- 
formed into  two. 

• 

In  sexual  reproduction  there  are  two  kinds  of  germ- 
cells,  the  egg_and  the  ^ermatozoan  respectively,  which 
take  part  in  producing  a  new  organism.  These  cells 
are  structurally  unlike  each  other  in  nearly  every  par- 
ticular, but  each  is  a  true  cell,  which  von  Kolliker  made 
clear  as  early  as  1841,  and  each  has  typically  the  same 
number  of  chromosomes  in  its  nucleus,  a  fact  more  re- 
cently determined  by  van  Beneden  in  1883. 

The   egg-cell   is   often   supplied   with   one   or   more 


222  GENETICS 

envelopes  of  protective  or  nutritive  function,  and  it  is 
usually  distended  with  stored  up  yolk,  in  consequence 
of  which  it  is  comparatively  large  and  stationary. 
The  result  is  that  whatever  locomotion  is  necessary  to 
bring  the  two  cells  together  for  union  devolves  upon 
the  sperm-cell.  Consequently  the  sperm-cells  are  prac- 
tically modified  into  nuclei  with  locomotor  tails  of  cyto- 
plasm, and  frequently,  in  addition,  with  some  structural 
modification  for  boring  a  way  into  the  egg-cell.  They 
are,  moreover,  much  more  numerous  than  the  egg-cells, 
so  that  although  many  go  astray,  never  fulfilling  their 
mission,  the  chances  are  nevertheless  good  that  some 
one  of  them  will  reach  the  egg  and  effect  fertilization. 

Ordinarily  only  one  sperm  enters  the  egg,  but  when 
several  succeed  in  penetrating  into  the  egg-cytoplasm 
only  one  proceeds  to  combine  with  the  egg  nucleus,  that 
is,  only  one  sperm  nucleus  is  normally  concerned  in  the 
essential  process  of  fertilization. 

It  was  formerly  thought  by  the  school  of  "ovists" 
that  in  fertilization  the  essential  process  is  a  stimula- 
tion of  the  all  important  egg  by  the  sperm.  The 
opposing  school  of  "spermists,"  on  the  other  hand,  re- 
garded the  egg  simply  as  a  nutritive  cell  the  function 
of  which  is  to  harbor  the  all  important  sperm.  It  is 
now  known  that  both  the  egg-  and  the  sperm-cell  are 
equally  concerned  in  fertilization,  which  consists  in  the 
union  of  their  respective  nuclei  within  the  cytoplasm  of 
the  egg. 


THE  CARRIERS  OF  THE  HERITAGE     223 

6.  MATURATION 

Certain  preliminary  changes  of  a  preparatory  na- 
ture, termed  maturation,  regularly  precede  the  union 
of  the  nuclei  of  the  two  sex-cells  in  fertilization. 

These  maturing  changes  result  in  reducing  the  outfit, 
of  chromosomes  in  each  sex-cell  to  one-half  the  originalx 
number,  a  process  which  is  necessary  in  order  to  main-/ 
tain   the   chromosomal   count   which   is    characteristic/ 
for  any  particular  species  and  which  is  known  to  exist 
unbroken  from  generation  to  generation.    If  there  were  ; 
no  such  reduction,  then  the  fertilized  egg,  formed  by 
the  union  of  egg  and  sperm  nuclei,  would  contain  double 
the  characteristic  number  of  chromosomes,  and  during 
the  formation  of  a  new  individual,  the  number  in  all 
the  cells  arising  by  mitosis  from  such  a  fertilized  egg 
would   likewise   be   double.      When    the   germ-cells    of 
such  individuals  unite  in  fertilization,  the  original  num- 
ber of  chromosomes  would  be  quadrupled,  and  so  on  in 
geometric  progression  throughout  subsequent  genera- 
tions.    In  1883,  too  late  for  Darwin  to  learn  of  it, 
van  Beneden  discovered  the  important  fact  that  the 
mature  germ-cells,  as  expected,  actually  contain  only 
half  the  normal  number  of  chromosomes. 

The  mature  egg-  or  sperm-cell,  with  half  its  normal 
number  of  chromosomes,  is  termed  a  gamete  (marry- 
ing cell),  while  the  fertilized  egg  which  is  formed  by 
the  union  of  two  gametes  (mature  egg-  and  sperm- 
cell),  and  which  consequently  has  the  characteristic 
number  of  chromosomes,  is  called  a  zygote  (yoked  cell). 

A   diagrammatic   representation   of   the  process   of 


GENETICS 


Fio.  58. — Diagram  to  show  typical  maturation  and  fertilization. 

maturation  is  shown  in  Figure  58.     The  number  of 
chromosomes  (not  shown  in  the  diagram)  remains  con- 


THE  CARRIERS  OF  THE  HERITAGE     225 

stant  in  each  germ-cell  respectively  until  the  division 
of  second  spermatocytes  into  spermatids  which  are 
subsequently  transformed  into  spermatozoa,  and  of  the 
second  oocytes  into  mature  eggs  and  second  polar  cells, 
when  it  is  reduced  to  one  half  the  normal  number.  As 
spermatozoan  and  mature  egg  unite  in  fertilization, 
the  original  number  of  chromosomes  is  restored  in  the 
fertilized  egg  (zygote). 

7.  FERTILIZATION 

The  stages  concerned  in  a  typical  case  of  fertiliza- 
tion, according  to  Boveri,  are  illustrated  in  Figures 
59  to  67. 

In  Figure  59  the  "head"  and  the  "middle  piece" 
of  the  sperm-cell  have  penetrated  into  the  egg  cyto- 
plasm, while  in  Figure  60  the  tail  of  the  sperm-cell 
has  become  lost  and  the  middle  piece,  which  furnished 
the  centrosome,  has  rotated  180°  so  that  it  lies  between 
the  nucleus,  or  head,  of  the  sperm-cell  and  that  of  the 
egg-cell.  Figure  61  shows  an  increase  in  the  size  of 
the  sperm  nucleus  and  a  division  of  the  centrosome  into 
two  parts  which  begin  to  migrate  towards  the  poles. 
This  process  of  polar  migration  of  the  centrosomes 
is  carried  further  in  Figure  62  as  well  as  the  increase  in 
the  size  of  the  sperm  nucleus,  until  in  Figure  63  the 
process  is  complete  so  that  the  centrosomes  have  as- 
sumed a  polar  position  and  the  sperm  nucleus  is  equal 
in  size  to  the  egg  nucleus  and  lies  in  contact  with  it. 
In  Figure  64  the  chromatin  network  of  the  two  nuclei 
has  formed  into  an  equal  number  of  chromosomes  which 


236 


GENETICS 


Fig.  59.  Entry  of  Sptrm  Fig.  60.  Loss  of.'SpenmTail 


Fig.  61.  Division  of 
Centrosome 


KB.  62.  Approach  of  $p,erm          Hg,  63.  Star  ease  of  Sjperm,          Fig.  S^Formation  of 
Nucleus  itPacleus  Chromosomes 


Big.  66.  Splitting  of  Chromosomes 


Fig,  66.  Anaphase 


Fag.  67.  Two-celled  Stage 

FIGS.  59-67.— Diagrams  illustrating  fertilization.    After  Boveri. 


THE  CARRIERS  OF  THE  HERITAGE     227 

in  each  case  is  half  the  number  characteristic  for  the 
species.  Figure  65  shows  the  complete  disappearance 
of  the  nuclear  membrane,  a  process  that  had  already 
begun  in  the  preceding  figure,  and  also  the  arrangement 
of  the  chromosomes,  connected  with  mantle  fibers,  in  the 
equatorial  plane  where  the  former  split  longitudinally. 
In  Figure  66,  when  the  half  chromosomes  thus  formed 
pull  apart  and  migrate  toward  the  poles,  the  segmenta- 
tion of  the  fertilized  egg  has  begun,  and  there  finally 
occurs,  as  shown  in  Figure  67,  the  two-celled  stage 
following  fertilization  in  which  each  cell  contains  the 
normal  number  of  chromosomes,  half  of  which  came 
from  the  egg  and  half  from  the  sperm. 

8.  PARTHENOGENESIS 

Fertilization  is  by  no  means  an  essential  process  in 
the  formation  of  a  new  individual,  even  in  those  ani- 
mals  which   produce   both   eggs    and    sperm.      Many 
animals    and    plants    reproduce    parthenogenetically, 
that  is,  the  egg-cell  may  develop  without  first  uniting   J 
with   a   sperm-cell.      In   these   instances   the   chromo- 
somes of  the  egg  are  not  halved  during  maturation,  and    \ 
the   offspring,    therefore,   have    the   same   number   of 
chromosomes    as    the   parent,    since   they    are    simply 
fragments  of  the  parent. 

Professor  Loeb,  by  the  use  of  certain  chemicals, 
has  succeeded  in  doing  artificially  what  apparently 
is  not  ordinarily  accomplished  in  nature,  namely,  mak- 
ing an  egg  that  normally  requires  fertilization  develop 
parthenogenetically. 


228  GENETICS 

V 

9.  THE   HEREDITARY  BRIDGE 

Whatever  may  ultimately  prove  to  be  determiners 
of  the  hereditary  characters  which  appear  in  successive 
generations,  it  is  obvious  that,  in  any  event,  such 
determiners  must  be  located  in  the  zygote,  that  is,  in 
the  fertilized  egg.  This  single  cell  is  the  actual  bridge 
of  continuity  between  any  parental  and  filial  genera- 
tion. Moreover,  it  is  the  only  bridge. 

In  the  majority  of  animals  the  egg  develops  en- 
tirely outside  of  and  independent  of  the  mother,  thus 
limiting  to  the  egg-cell  itself  all  possible  maternal 
contributions  to  the  offspring.  Although  there  is 
abundant  evidence  that  half  of  the  filial  characteristics 
come  from  the  male  parent,  the  only  actual  fragment 
of  the  paternal  organism  given  over  to  the  new  indi- 
vidual is  the  single  sperm-cell,  which  unites  with  the  egg 
in  fertilization,  and  the  whole  of  this  even  is  not  usually 
concerned  in  the  process  of  fertilization.  The  entire 
factor  of  heritage  is  packed  into  the  two  germ-cells 
derived  from  the  respective  parents  and,  in  all  prob- 
ability, into  the  nuclei  of  these  germ-cells,  since  the 
nuclei  are  apparently  the  only  portions  of  these  cells 
that  invariably  take  part  in  fertilization.  To  the  new 
individual  developing  by  mitosis  from  the  fertilized  egg 
into  an  independent  organism,  the  factors  of  environ- 
ment and  response  referred  in  to  Figure  1  are  subse- 
quently added. 

When  it  is  remembered  that  the  human  egg-cell 
is  only  about  Vizsth  of  an  inch  in  diameter,  a  gigantic 
size  as  compared  with  that  of  the  human  sperm-cell, 


THE  CARRIERS  OF  THE  HERITAGE     229 

and,  furthermore,  when  one  passes  in  rapid  review 
the  marvelous  array  of  characteristics  which  make  up 
the  sum  total  of  what  is  obviously  inherited  in  man, 
the  wonder  grows  that  so  small  a  bridge  can  stand 
such  an  enormous  traffic.  A  sharp-eyed  patrol  of 
this  bridge  as  the  strategic  focus  of  heredity  is  proving 
to  be  one  of  the  most  effective  points  of  attack  in  the 
entire  campaign  of  genetics. 

10.  THE  CHROMOSOME  THEORY 

Certain  investigators,  who  seek  a  morphological  basis 
for  heredity,  regard  the  chromosomes  as  the  carriers 
of  the  heritage ;  in  other  words,  as  the  source  of  the  de- 
terminers of  ontogeny  or  the  effective  factors  in  the 
process  of  differentiation. 

A  few  of  the  grounds  for  this  theory  are  briefly 
indicated  below. 

First:  In  spite  of  the  great  relative  difference  in 
size  between  the  egg-cell  and  the  sperm-cell,  in  heredity 
the  two  are  practically  equivalent,  as  has  been  re- 
peatedly shown  by  making  reciprocal  crosses  between 
the  two  sexes.  The  only  features  that  are  apparently 
alike  in  both  the  germ-cells  are  the  chromosomes.  The 
inference  is,  therefore,  that  they  contain  the  determiners 
which  are  the  causal  factors  for  the  equivalence  of 
adult  characters  in  heredity.  The  existence  of  an  extra 
chromosome  in  probable  connection  with  the  matter 
of  sex  is,  as  will  be  pointed  out  later,  an  exception  to 
the  exact  chromosome  equivalence  of  the  two  sexes, 
which  only  goes  to  strengthen  the  supposition  that  the 


230  GENETICS 

chromosomes  are  the  carriers  of  hereditary  qualities 
since  extra  chromosomes  are  always  associated  with 
the  character  of  sex. 

Second:  The  process  of  maturation,  which  always 
results  in  halving  the  chromosome  material  of  the 
germ-cells  as  a  preliminary  step  to  fertilization,  is  a 
series  of  complicated  manoeuvers  not  practised  by  other 
cells.  During  this  process  no  other  part  of  the  cells 
appears  to  play  so  consistent  and  important  a  role 
as  the  chromosomes.  Provided  they  act  as  hereditary 
carriers,  their  peculiar  behavior  during  maturation  is 
just  what  is  needed  to  bring  together  an  entire  comple- 
ment of  hereditary  determiners  out  of  partial  contri- 
butions from  two  parental  sources. 

Third:  Sometimes  abnormal  fertilization  occurs,  as 
in  the  case  when  two  or  more  sperm-cells,  instead  of 
one,  enter  the  egg  cytoplasm  and  unite  with  the  egg 
nucleus.  This  unusual  performance  has  been  artifi- 
cially induced  by  chemical  means  in  the  case  of  sea- 
urchins'  eggs.  The  fertilized  egg,  or  zygote,  thus 
formed  with  an  excess  of  male  chromosomes,  results  in 
the  development  of  abnormal  larvae.  It  is  thought  that 
a  causal  connection  may  exist,  therefore,  between  the 
additional  male  chromosomes  in  the  fertilized  ovum  and 
the  abnormalities  of  the  progeny. 

Fourth:  The  fact  that  chromosomes  may  retain 
their  individuality  throughout  the  complicated  phases 
of  mitosis,  as  has  been  proven  in  some  instances,  agrees 
with  the  corresponding  fact  that  certain  characteristics 
of  the  somatoplasm  maintain  their  individuality  from 
generation  to  generation. 

Moreover,  certain  chromosomes  in  the  fertilized  egg 


THE  CARRIERS  OF  THE  HERITAGE 

have  been  identified  with  particular  features  in  the 
adult  developing  from  that  egg.  Tennent  summarizes 
his  work  on  Echinoderms  (1912)  by  the  statement 
that  from  a  knowledge  of  the  chromosomes  in  the 
parental  germ-cells,  particular  characters  in  the  adult 
hybrids  may  be  predicted,  and,  conversely,  that 
from  the  appearance  of  sexually  mature  hybrids  the 
character  of  certain  chromosomes  in  their  germ-cells 
may  be  predicted. 

Again,  the  correlation  of  a  particular  chromosome  in 
the  germ-cells  with  a  definite  adult  character,  namely 
sex,  has  been  repeatedly  demonstrated  in  connection 
with  the  so-called  "extra  chromosome"  to  which  refer- 
ence has  already  been  made. 

Fifth:  Finally,  excellent  evidence  of  a  definite  causal 
connection  between  certain  chromosomes  of  the  germ- 
cells  and  particular  somatic  characters  has  been  fur- 
nished by  certain  critical  experiments  upon  the  eggs 
of  sea-urchins.  Boveri  found  that  he  was  able  in  some 
instances  to  shake  out  the  nuclei  bodily,  chromosomes 
and  all,  from  the  mature  eggs  of  the  sea-urchin, 
Splicer  echinus,  and  when  there  was  added  in  sea  water  to 
such  enucleated  eggs  the  sperm-cells  of  an  entirely 
different  genus  of  sea-urchin,  namely,  Echwws,  the 
Echinus  sperm-cells  entered  the  Sphcerechmus  eggs, 
which  had  been  robbed  of  their  nuclei,  and  from  this 
peculiar  combination  larvae  developed  which  exhibited 
only  Echinus  characters! 

Such  cumulative  circumstantial  evidence  as  the  fore- 
going has  convinced  many  that  in  the  chromosomes  we 
have  visibly  before  us  the  carriers  of  heredity. 

In  any  event  the  supposition  that  the  chromosomes, 


GENETICS 

with  certain  chemical  reservations,  are  the  morpho- 
logical carriers  of  the  heritage,  forms  an  excellent 
working  hypothesis,  and  this  chapter  may  suitably  be 
closed  with  a  quotation  from  Professor  Wilson,  whose 
brilliant  work  in  the  entire  field  of  cytology  makes  it 
possible  for  him  to  speak  with  authority.  "In  my 
view  studies  in  this  field  are  at  the  present  time  most 
likely  to  be  advanced  by  adopting  the  comparatively 
simple  hypothesis  that  the  nuclear  substances  are  actual 
factors  of  reaction  by  virtue  of  their  specific  chemical 
properties ;  and  I  think  that  it  has  already  helped  us 
to  gain  a  clearer  view  of  some  of  the  most  puzzling 
problems  of  genetics.'* 


CHAPTER  XI 

THE  ARCHITECTURE  OF  THE  GERMPLASM 

1.  DROSOPHILA,  THE  BIOLOGICAL  CINDERELLA 

JUST  as  the  bacteriologist  firmly  believes  that  guinea- 
pigs  were  specially  created  for  serological  experimenta- 
tion, so  the  geneticist  has  come  to  realize  that  the 
banana-fly,  Drosophila  melanogaster,  to  which  repeated 
reference  has  already  been  made,  was  designed  for 
disclosing  the  secrets  of  the  "architecture  of  the  germ- 
plasm"  (Weismann). 

This  tiny  ubiquitous  fly  (Fig.  32),  which  hovers 
around  bruised  fruit  without  regard  to  place,  is  so 
small  and  harmless  that  it  does  not  even  qualify  as 
a  pest.  It  has  proved,  nevertheless,  to  be  a  veritable 
bonanza  to  the  geneticist.  It  has  many  well-defined 
characters  that  can  be  observed  under  the  microscope 
and  it  lives  successfully  upon  a  bit  of  banana  in  a  milk 
bottle  plugged  with  cotton.  Every  ten  or  eleven  days 
a  pair  produces  two  to  three  hundred  descendants  that 
in  turn  are  ready  to  produce  similar  families  of  their 
own  so  that  the  investigator  who  begins  with  them  needs 
to  be  an  expert  bookkeeper  in  order  to  be  able  to  record 
his  results.  Although,  like  Cinderella,  Drosophila 
comes  from  the  humble  environment  of  the  garbage  can, 
yet  this  fly  has  easily  outstripped  all  its  sister  competi- 

233 


GENETICS 

v 

tors  for  genetical  honors,  until  to-day  it  stands  prob- 
ably as  the  most  famous  experimental  organism  in  the 
whole  world. 

Prof.  T.  H.  Morgan  of  Columbia  University  is  the 
most  conspicuous  leader  in  the  investigation  of  Droso- 
phila.  In  his  laboratory  -over  ten  millions  of  these 
animals,  which  literally  "breed  like  flies,"  have  passed  in 
review  under  the  microscope  while  pedigrees  of  over 
three  hundred  generations  have  been  obtained  and 
recorded.  In  no  other  plant  or  animal  has  the  remark- 
able parallelism  between  the  segregation  of  Mendelian 
characters  in  experimental  breeding  and  the  behavior 
of  the  chromosomes  been  so  completely  demonstrated. 

2.  LINKAGE 

Drosophlla  has  only  four  pairs  of  chromosomes  al- 
though more  than  three  hundred  different  characters 
have  been  found  in  the  flies  themselves,  a  fact  which 
makes  it  at  once  evident  that  many  genes,  or  character- 
determiners,  must  be  located  together  in  each  chromo- 
some. 

Experimental  breeding  of  Drosophila  shows  that 
there  is  not  always  complete  independent  assortment  of 
the  different  characters  that  enter  into  a  cross,  as 
Mendel  found  to  be  true  for  the  different  characters  of 
peas  with  which  he  experimented. 

Genes  located  together  in  any  one  chromosome  are 
likely  to  stay  together  during  the  conjugation  of  the 
chromosomes  and  the  subsequent  separation  of  the 
members  of  homologous  pairs  in  the  process  of  matura- 


ARCHITECTURE  OF  THE  GERMPLASM    235 

tion.  This  hanging  together  of  neighboring  genes  of 
the  same  chromosome  throughout  the  complicated  pro- 
cess of  meiosis  is  termed  linkage. 

It  is  extremely  fortunate  for  the  evolution  of  our 
knowledge  of  the  mechanism  of  heredity  that  Mendel 
happened  to  work  upon  characters  located  in  separate 
chromosomes  and  so  was  able  to  establish  the  law  of 
the  independent  segregation  of  unit  characters  before  the 
apparent  contradiction,  that  is,  linkage,  became  known. 
If  he  had  come  upon  the  confusing  phenomenon  of  linkage 
first,  the  discovery  of  the  laws  of  Mendelism,  in  all  prob- 
ability, would  have  been  long  delayed. 

Bateson  and  Punnett  called  attention  to  linkage  as 
early  as  1906  under  the  name  of  "coupling"  in  the  case 
of  certain  characters  of  sweet  peas.  A  vague  general 
knowledge  of  many  groups  of  correlations,  such  as 
deafness  and  defective  teeth  going  along  with  blue 
eyes  and  albinism  in  cats,  had  for  a  long  time  existed. 

In  Drosophila,  the  brilliant  and  extensive  investiga- 
tions of  Morgan  and  his  co-workers  have  resulted  in 
definitely  placing  something  like  two  hundred  characters 
in  four  linkage  groups  corresponding  to  four  pairs  of 
chromosomes.  The  limitation  of  linkage  groups  to  the 
number  of  chromosome  pairs  found  in  the  organism 
is  proving  to  be  one  of  the  fundamental  principles  of 
heredity. 

Moreover,  it  has  been  shown  by  reciprocal  crosses 
that  linkage  when  it  occurs  is  not  due  to  some  relation 
per  se  between  the  genes  but  simply  to  the  fact  that 
the  linked  genes  chance  to  lie  together  in  the  same  chro- 
mosome. In  other  words,  if  two  characters  enter  a 


236 


GENETICS 


cross  together  from  one  parent  they  will  stay  together 
in  the  offspring,  and  if  they  enter  from  separate  parents 
they  remain  separate  in  the  offspring. 

t 


ft* 


ft* 


ft* 


GL 


bv 


GL 

Gv 

bL 

bv 

bv 

bv 

bv 

bv 

GL 

Gv 

bL 

bv 

bv 

bv 

bv 

bv 

GL 

Gv 

bL 

bv 

bv 

bv 

bv 

bv 

GL 

Gv 

bL 

bv 

bv 

bv 

bv 

bv 

FIG.  68. — Checkerboard  to  show  the  result  of  crossing  a  gray-long, 
black-vestigial  hybrid  fly  back  to  a  black-vestigial  recessive. 
G  =  gray-body ;  L  =  long-wings ;  b  =  black-body ;  v  =  ves- 
tigial wings. 


3.  THE  MODUS  OPERANDI  OF  LINKAGE 

The  way  linkage  works  out  may  best  be  made  clear 
by  illustrations  from  Morgan.  When  an  ordinary 
wild-type  fly  with  gray  body  and  long  wings  is  crossed 
with  a  fly  showing  the  two  mutations  of  black  body  and 


ARCHITECTURE  OF  THE  GERMPLASM    287 


vestigial  wings,  the  hybrids  of  the  first  generation  are 
all  like  the  wild-type  parent  because  gray  body  and 
long  wings  are  dominant  over  black  body  and  vestigial 
wings. 


Parents 


Gametes 


Gamete* 


FIG.  69.— Typical  linkage  in  Drosophila.     Symbols  as  in  Fig.  68. 
Data  from  Morgan. 

When  a  male  of  one  of  these  hybrid  flies  is  crossed 
back  with  a  recessive  black-vestigial  female,  if  Mendelian 
segregation  took  place  there  would  be  four  possible 
kinds  of  offspring,  as  shown  in  Figure  68,  viz.,  gray- 


238  GENETICS 

long;  gray-vestigial;  black-long  and  black-vestigial. 
The  actual  experiment,  however,  shows  but  two  classes 
of  offspring,  viz.,  gray-long  and  black- vestigial,  like  the 
two  grandparents  (Fig.  69).  In  other  words,  gray- 
body  and  long-wings  entering  the  cross  from  one  parent 
stay  linked  together  as  do  also  black-body  and  vestigial 
wings.  The  method  of  crossing  the  hybrid  back  to 
the  recessive  is  the  common  procedure  in  order  to  bring 
out  what  is  latent  in  the  hybrid,  for  the  recessive,  since 
it  does  not  dominate  or  conceal  anything,  allows  what- 
ever is  present  in  the  hybrid  being  tested  to  appear. 
The  Mendelian  practice  of  crossing  the  FI  hybrids 
together  tends  to  conceal  linkage  and  perhaps  has 
prevented  its  earlier  recognition. 

The  reciprocal  cross  is  shown  in  Figure  70.  In 
this  case  likewise,  whatever  goes  in  together  comes 
out  together  and  no  new  combinations  appear. 

4.  CROSS-OVER 

When  a  gray-bodied  long-winged  female  hybrid,  such 
as  is  produced  by  crossing  gray-long  and  black-vesti- 
gial together  in  the  preceding  experiment,  is  crossed 
back  to  a  recessive  black-vestigial  male,  there  are  pro- 
duced four  kinds  of  offspring,  gray-long  and  black- 
vestigial  like  the  grandparents  and  two  new  combina- 
tions, gray-vestigial  and  black-long.  These  four  types 
of  F2  are  what  would  be  expected  upon  free  assort- 
ment of  all  the  gametes  and  they  should  all  occur  in 
equal  numbers  or  in  the  proportion  of  1  : 1  : 1  : 1.  See 
Figure  68.  Instead,  as  an  actual  result  of  extensive 


ARCHITECTURE  OF  THE  GERMPLASM    239 

crosses  of  this  kind,  Morgan  obtained  41.5%  each  of 
gray-long    and    black-vestigial    and    8.5%     each    of 


'Parents 


Gametes 


Gametes 


FIG.  70. — Typical  linkage  in  Drosophila.     Reciprocal  to  the  case 
shown  in  Fig.  69.    Data  from  Morgan. 

the  new  combinations  of  black-long  and  gray-vestigial. 
(See  Figure  71.)  The  new  combinations  represent 
cross-overs  or  breaks  in  the  linkage  of  the  genes  within 
the  chromosomes. 

Although  this  superficially  resembles  the  free  assort- 
ment or  segregation  of  typical  Mendelian  crosses,  it 


240 


GENETICS 


Parents 


Gametes 


Gametes 


Expectation  ] 
tnMendelian  }• 
Assortment  J 


.Artwtl  Results-    41. 5  # 


Linkage 


Cross-overs 


FIG.  71. — Typical  cross-over  in  Drosophila.    Symbols  as  in  Fig.  68. 
Data  from  Morgan. 

is  quite  a  different  thing  since  Mendelian  segregation 
involves  whole  chromosomes  while  cross-overs  involve 
only  parts  of  chromosomes.  The  percentage  too  of  the 


ARCHITECTURE  OF  THE  GERMPLASM 

different  classes  resulting  in  the  F2  generation  from 
hybrids  is  different  in  typical  Mendelian  segregation 
and  in  cross-overs. 

Furthermore,  the  percentage  of  cross-overs  varies  in 
different  crosses.  For  example,  when  white-eyed  yel- 
low-bodied flies  are  crossed  with  normal  wild-type  red- 
.  eyed  gray-bodied  individuals,  the  resulting  hybrids  re- 
semble wild  red-eyed,  gray-bodied  flies.  When  such  a 
female  hybrid  is  crossed  back  to  a  recessive  white-eyed 
yellow-bodied  male,  the  offspring  show  only  one  per- 
cent of  cross-overs,  that  is,  white-eyed,  gray-bodied  and 
red-eyed,  yellow-bodied  individuals,  and  99%  of  linkage, 
that  is,  white-eyed,  yellow-bodied  and  red-eyed,  gray- 
bodied  (Fig.  72). 

Another  percentage  of  cross-over,  that  between 
white-eye  and  miniature-wing  was  found  to  be  33.  It  is 
obvious  that  in  any  case  the  cross-over  will  never  ex- 
ceed 50%. 

Jennings  has  said:  "The  studies  of  'crossing-over' 
promise  to  bring  us  into  closer  touch  with  the  actual 
,  details  of  the  hereditary  mechanism  than  any  other 
phenomena  now  under  examination." 

5.  How  DO  CROSS-OVERS  OCCUR? 

In  germ-cells  before  maturation,  homologous  mater- 
nal and  paternal  chromosomes  pair  off  and  usually  come 
to  lie  side  by  side.  This  is  the  phenomenon  of  syndesls 
or  conjugation.  During  this  temporary  contact  there 
seems  to  be  an  opportunity  for  such  an  exchange  of 
parts  as  cross-over  breeding  demonstrates  does  actually 


99$  \% 

FIG.  72. — A  case  of  one  percent  cross-over  in  Drosophila.  Gray- 
body  and  red-eyes  are  represented  by  stippling  and  solid  black 
respectively.  Yellow-body  and  white-eyes  are  unshaded.  After 
Sharp,  from  Morgan's  data. 

242 


ARCHITECTURE  OF  THE  GERMPLASM    243 


occur.  Syndesis  has  been  repeatedly  observed  and 
sometimes  two  chromosomes  are  seen  even  to  twist 
about  each  other.  When  separation  comes  after  this 
embrace  the  two  original  chromosomes  may  simply 
unwind  and  so  regain  their  former  condition  unchanged, 
or  they  may  break  and  fuse  in  such  a  way  that  one  (A) 
has  a  part  of  the  other  (B),  and  the  remaining  parts 
show  a  corresponding  fusion,  as  indicated  in  Figure  73. 
This  is  the  chromosomal  explanation  (Chiasmatype 


FIG.   73. — Diagram  to  show  cross-over  between  two  homologous 
chromosomes.     After  Muller. 

theory    of    Janssens)    of    the    cross-over    phenomena 
known  to  the  experimental  breeder. 

6.  INTERFERENCE 

The  varying  percentages  of  cross-overs  between  dif- 
ferent pairs  of  genes  led  Morgan  and  his  associates  to 
attempt  the  localization  of  genes  within  the  chromo- 
somes. The  idea,  as  suggested  by  Bridges  in  1914,  is 
simply  this,  that  the  farther  apart  two  genes  are  in 
the  chromosome  the  more  likely  they  are  to  cross 
over  and  to  exchange  places  with  their  homologous 
genes  during  syndesis. 


244 


GENETICS 


Of  course  if  they  lie  very  close  together  in  the  chro- 
mosome they  are  apt  to  be  found  finally  on  the  same  side 
regardless  of  the  twisting  of  the  paternal  and  maternal 
chromosomes  about  each  other.  This  is  evident  in 

_       Figure   74   where   the   invisible 

genes  are  represented  hypo- 
thetically  by  letters  placed 
within  the  chromosomes.  Cross- 
over is  more  likely  to  occur  be- 
tween A  and  E  which  lie  at  the 
extremes  of  chromosome  I  than 
between  A  and  B  which  are 
closer  together. 

Again,  when  genes  lie  close 
together  they  theoretically  in- 
terfere with  the  crossing  over 
of  neighboring  genes  as  pointed 
out  by  Muller  and  confirmed  by 
subsequent  breeding  experi- 
ments. In  Figure  74,  for  ex- 
ample, if  crossing-over  took 
place  between  the  pairs  Cc  and 
Dd,  breaking  the  linkage  be- 
tween C  and  D  and  between  c 
and  d,  it  would  prevent  another 

break  of  linkage  between  BC  and  be.  This  is  the  phe- 
nomenon of  interference.  It  follows  that  the  nearer 
together  two  pairs  of  genes  involved  in  cross-over  are 
located,  the  greater  will  be  the  interference. 


FIG.  74. — Interference. 
Two  homologous  chro- 
mosomes during  syn- 
desis.  When  there  is 
a  cross-over  between 
Cc  and  Dd,  it  inter- 
feres with  another 
cross-over  near  by  be- 
tween Cc  and  Bb. 


ARCHITECTURE  OF  THE  GERMPLASM    245 

7.  THE  ARRANGEMENT  OF  THE  GENES 

Morgan  assumes  that  if  one  per  cent  of  cross-overs 
occurs  this  may  be  made  to  represent  one  arbitrary 
unit  of  distance  between  the  two  genes  in  question. 
Haldane  proposes  to  call  this  unit  of  cross-over  a 
morgan.  In  the  illustration  of  black-body  and  vestigial- 
wing  where  there  was  17%  of  cross-over  it  is  assumed 
that  the  genes  for  these  two  characters  are  17  units,  or 
morgans,  apart  in  the  chromosome. 

Following  up  this  fertile  idea  it  becomes  possible 
even  to  map  the  location  of  the  genes  in  the  chro- 
mosomes. Sturtevant  was  the  first  to  make  such  a 
map  for  the  genes  in  the  "sex  chromosome"  of  Droso- 
phila. 

This  has  been  followed  by  maps  of  the  other  chro- 
mosomes, after  breeding  a  total  of  several  million  flies 
and  analyzing  the  data  which  include .  altogether  the 
behavior  of  over  a  hundred  different  genes. 

The  relative  location  of  the  genes  has  been  determined 
by  the  following  method.  If  for  example  two  genes, 
A  and  B,,  upon  breeding  back  to  the  recessive  show  5% 
of  cross-overs  with  a  and  b,  while  B  and  C  show  20% 
with  their  allelomorphs,  b  and  c,  then  when  A  and  C 
are  bred  together  with  a  and  c,  they  should  give 
either  the  sum  (5  +  20  =  25%)  or  the  difference 
(20  —  5  =  15%)  of  cross-overs. 

For  example,  in  an  actual  experiment,  yellow-body 
and  white-eye  gave  1.2%  cross-overs  while  white-eye 
and  bifid-wing  gave  3.5%  cross-overs.  When  yellow- 
body  and  bifid-wing  were  tested  they  met  the  expecta- 


246  GENETICS 

tion  and  gave  4.7%,  or  the  sum  of  the  other  two  per- 
centages, as  shown  in  Figure  75. 

If  upon  breeding  yellow  and  bifid  a  percentage  of 
2.3%  had  been  obtained  instead  of  4.7%  as  was  ac- 
tually found,  then  the  order  of  the  genes  would  have 
been  yellow-bifid-white  instead  of  yellow-white-bifid. 

In  the  eloquent  frontispiece  of  The  Mechanism  of 
Mendelian  Heredity,  by  Morgan,  Sturtevant,  Bridges 
and  Muller,  there  are  drawn  four  straight  parallel  lines 
representing  the  "chromosome  maps"  of  Drosophila 

yellow 

0 

white  4.7% 

9 

bifid' 

FIG.  75. — An  illustration  of  the  proof  of  gene-localization  from 
cross-over  percentages  obtained  by  breeding. 

as  known  in  1915.  It  is  doubtful  if  in  any  book  there 
may  be  found  four  straight  lines  that  mean  so  much. 
The  work  of  gene-localization  is  quite  comparable  to 
that  done  by  mathematicians  and  astronomers  in  deter- 
mining the  distances  that  separate  the  stars  in  the 
heavens  from  each  other  and  is  perhaps  equally  in- 
comprehensible to  the  layman.  In  gene-localization  it 
is  the  infinitely  small  instead  of  the  infinitely  great 
that  one  must  observe.  When  it  is  remembered  that 
Drosophila  is  a  very  tiny  fly;  that  occupying  only  a 
small  part  within  its  abdomen  are  paired  reproductive 
organs;  that  each  of  these  reproductive  organs  in  the 


13.7 
16.7- 


20.0. 


43.0- 


65.0- 


^ruby 


cross-vcinless 


club 


•inged 


tan 


vermilion 
tt0r:brt*tZM 


sable 


44.4-   •  garnet 


63.6- 


3!6-  -  small-eye 
9.5-   s/twed 


-cleft 


-2.0-r  telegraph 
star 
aristaless 


4.0- 


9.0- 


13.0- 
14.0- 


220J- 


28.0- 

ae.o- 


83.0- 
35  Jr 


48.5- 


B8.0 


expanded 


0«K 

pink-wing 

streak 

eream-b 

dacha 
ski 


apterous 
purple 


eafranin 


61.0  -  trefoil 

65.0-  •  vestigial 

66.6.  -  telescope 

67.0"  Xdoaft 


76.0"  -^dachsous 
77.0   •  roe^" 


0.0-1-  roughoid 


26.3. 


70. 


89. 


95.< 


beaded 


88.0   -  ftwB^V 


«6.o4-  pwpteoid 

are 

plexus , 
lethal  lla 
brown 

•blister  t 


FIG.  76. — Chromosome  maps  of  Drotophila.    After  Sharp. 
247 


1 


eyeless± 


.0.0 


divergent 


Oitnoraxota 
69.0-  -  alass 
en  BJO.O:  -kidney 

m*mS  ^jz%ad 

63.5    -  delta 
65.6-  ->.JiatrZess 


o5  -f» 


an 


72.0-  •  white  oeeUi 


86.6    -  rough 


f\ 


248  GENETICS 

male  is  made  up  of  several  tubules;  that  within  these 
tubules  may  eventually  be  found  the  sperm  cells  with 
plenty  of  room  to  move  about;  that  within  a  single 
sperm  cell  is  the  nucleus ;  that  after  half  of  the  contents 
of  the  nucleus  has  been  disposed  of  there  remain  four 
chromosomes ;  that  within  each  chromosome  beyond  the 
range  of  vision  there  are  hundreds  of  genes  and  that  it 
has  been  possible  in  a  single  chromosome  to  determine 
not  only  the  relative  arrangement  of  over  thirty  genes 
but  also  to  find  out  the  relative  distance  between  these 

Fewle  Male  Senes>  *  wil1  be   realized 

that   the   analysis   of   the 

germplasm     has     gone     a 
long  way. 

In  Figure  76,  taken  from 

FIG.  77.— The  chromosomes  of      Sharp's   "Introduction   to 

Drosophila    melanogaster. 

After  Bridges.  Cytology,"  are  repre- 

sented the  four  chromo- 
some maps  of  Drosophila  corrected  to  November,  1920. 
The  four  visible  chromosomes  of  Drosophila  corre- 
spond to  the  four  linkage  groups  of  characters  obtained 
by  experimental  breeding  and  it  is  a  striking  fact  that 
no  character  has  yet  appeared  that  cannot  be  assigned 
to  one  of  these  four  linkage  groups.  The  relative 
length  of  the  four  "maps,"  which  has  been  determined 
from  the  carefully  worked-over  data  acquired  by  years 
of  riotous  breeding  for  cross-overs,  agrees  remarkably 
with  the  relative  differences  in  the  actual  size  of  the 
chromosomes  as  measured  under  the  microscope.  The 
four  pairs  of  chromosomes  in  a  male  Drosophila  mel- 
anogaster are  represented  in  Figure  77. 


ARCHITECTURE  OF  THE  GERMPLASM    249 

8.  LINKAGE  IN  OTHER  ORGANISMS 

The  phenomenon  of  linkage  has  already  been  observed 
in  various  other  organisms  besides  Drosophila.  Even 
in  Mendel's  classic  peas  White  demonstrated  four  link- 
age groups  of  characters  and  seven  pairs  of  chromo- 
somes. It  is  doubtful  if  Mendel  himself  ever  heard  of 
chromosomes  for  he  died  in  1886  and  Boveri's  pioneer 
work  on  chromosomes  had  only  then  recently  appeared. 
A  list  of  a  few  of  the  organisms  in  which  linkage  has 
been  reported  is  given  below. 

Organisms  that  show  linkage  Author 

Sweet  pea  Bateson  and  Punnett 

Snapdragon,  Wheat  Baur 

Primula    Altenberg.   Gregory 

Maize    Emerson.  Breggar. 

Lindstrom.  Jones 

Tomato    Jones 

Beans.    Oats Surface 

Evening  primrose Shull 

Drosophila  virilis  1 

busckii  I Metz 

repletaj 

Silkworms    Tanaka 

Grouse  locust  Nabours 

Pigeon    Cole  and  Kelly 

Rat.    Mouse .Castle  and  Dunn 

Rat    Ibsen 

Rabbit ...Castle 


CHAPTER  XII 

SOMATOGENESIS 
1.  THE  HEREDITARY  TUNNED 

The  earlier  studies  in  heredity  were  concerned  with 
the  comparison  of  successive  individuals,  or  somato- 
plasms.  This  phenotypic  method  has  attained  a  con- 
siderable degree  of  success  through  the  analysis  afforded 
by  Mendelism. 

A  different  and  still  more  recent  method  of  attack 
upon  the  problem  of  heredity  deals  not  with  individuals 
but  with  chromosomes  which  are  generally  acknowledged 
to  be  the  living  springs  from  which  flow  the  streams 
of  inheritance.  Such  an  intensive  cytological  study 
of  the  germplasm  has  revealed  a  mechanism  that  ex- 
plains to  a  marvelous  extent  the  results  of  the  experi- 
mental breeder. 

The  demonstration  of  the  parallel  between  the  be- 
havior of  the  germplasm  as  seen  in  the  chromosomes 
and  the  performance  of  the  somatoplasm  as  exhibited 
in  the  end  results  of  experimental  breeding,  is  one  of 
the  most  impressive  scientific  achievements  of  our  times. 

There  is  an  undoubted  causal  connection  between 
the  genotype  and  the  phenotype  at  the  extremes  of  the 
hereditary  pageant  but  between  these  extremes,  that  is, 
between  the  fertilized  egg  and  the  adult,  investigators 
are  as  yet  by  no  means  as  confident  or  well-informed. 

250 


SOMATOGENESIS  251 

It  is  as  if  heredity  was  represented  by  a  long  under- 
ground tunnel.  We  are  in  the  light  at  either  end 
and  have  made  out  to  a  considerable  degree  the  details 
at  the  entrance  and  exit,  but  we  are  still  largely  in 
darkness  throughout  the  passage-way  itself. 

The  science  of  embryology  has  given  us  a  series  of 
flash-light  pictures  of  what  goes  on  in  the  tunnel  of 
development  but  of  necessity  its  contribution  has  been 
largely  morphological.  Consequently  the  geneticist 
still  awaits  some  torch-bearer  who  will  reveal  how  an 
invisible  gene  within  a  chromosome  can  give  form  and 
substance  to  a  definite  visible  unit  character  in  an  or- 
ganism. Probably  genetics  has  contributed  more  to 
embryology  than  embryology  to  genetics  in  the  past  but 
it  is  quite  likely  that  the  account  will  be  more  than 
balanced  in  the  future. 

The  way  in  which  germ-cells  come  by  their  potent 
hereditary  components,  rather  than  how  they  make  use 
of  them,  has  been  the  first  and  most  natural  problem 
to  engage  the  attention.  The  solution  which  satisfies 
most  biologists,  who  have  considered  the  evidence,  has 
been  found  in  the  idea  of  the  contmmty  of  the  germ- 
plasm,  that  is,  that  hereditary  genes  are  not  the  product 
and  result  of  the  body  carrying  them  but  are  lineal 
descendants  of  ancestral  genes  which  have  been  housed 
temporarily  in  other  bodily  domiciles  in  the  past. 

The  familiar  miracle  of  how  hereditary  genes  work 
together  to  produce  a  new  plant  or  animal  is  farther 
from  a  satisfactory  solution,  yet  there  is  no  doubt  that 
some  of  the  impending  great  discoveries  in  genetics  are 
sure  to  be  exactly  in  this  field. 


25*  GENETICS 

2.  PREFORMATION  AND  EPIGENESIS 

How  does  germplasm  transmute  into  somatoplasm? 

Historically  there  have  been  two  conspicuous  at- 
tempts to  solve  the  riddle  of  differentiation,  neither  of 
which  gives  intellectual  satisfaction  any  longer  in  the 
light  of  what  is  known  to-day. 

The  first  held  sway  in  the  17th  and  18th  centuries 
under  the  guise  of  the  preformation  theory  which  as- 
sumes that  development  is  simply  the  unfolding  and  en- 
larging of  what  was  already  present  in  the  germ  in 
miniature.  This  has  been  called  the  theory  of  "emboite- 
ment"  or  "infinite  encasement,'5  because,  not  only  is 
the  miniature  plant  or  animal  supposed  to  be  packed 
within  the  germ-cell  like  the  embryo  plant  between  the 
cotyledons  of  the  bean  seed,  but  within  each  miniature 
also  it  is  supposed  that  the  next  generation  is  encased, 
and  the  next,  ad  mfinitum.  Aided  by  a  poor  microscope 
and  a  good  imagination  the  theory  of  preformation 
was  carried  to  such  an  extreme  that  a  mannikin  or 
"homunculus"  was  actually  figured  by  Hartsoeker 
seated  within  the  head  of  a  human  spermatozoan ! 

The  second  attempt  to  solve  the  riddle  of  develop- 
ment resulted  in  the  theory  of  eplgenesis  which  goes 
to  the  other  extreme,  maintaining  that  organization 
gradually  appears  out  of  an  absolutely  simple  undiifer- 
entiated  germ.  This  theory  had  its  most  influential  ex- 
position in  "Theoria  Generationis"  by  C.  F.  Wolff  in 
1759.  "The  mistake  in  the  doctrine  of  preformation 
was  in  supposing  that  germinal  parts  were  of  the  same 
kind  as  adult  parts;  the  mistake  of  epigenesis  was  in 
maintaining  a  lack  of  specific  parts  in  the  germ." 


SOMATOGENESIS  253 

(Conklin).     Neither  of  these  two  conceptions  is  in  ac- 
cordance with  the  facts  as  known  to-day. 


3.  WHAT  is  SOMATOGENESIS? 

Development  is  not  simply  the  unfolding  or  assort- 
ment of  what  is  already  present  in  the  germ  nor  is  it 
the  miraculous  writing  of  something  new  upon  a  clean 
slate.  Rather  it  is  the  orderly  initiation  and  sequence 
of  new  structures  and  functions  conditioned  by  the 
interaction  of  the  germinal  elements  present  in  the  egg 
or  ovule. 

Thus  somatogenesis  is  the  study  of  the  emergence  of 
bodily  structure  out  of  hereditary  sources.  Like  the 
evolution  of  species,  which  has  so  enthralled  the  minds 
of  thinking  men,  somatogenesis  in  a  parallel  way  is  the 
evolution  of  the  individual.  No  doubt  each  of  these 
epic  histories  will  eventually  furnish  the  key  and  vo- 
cabulary to  the  other. 

Both  somatogenesis  and  garnet ogenesis,  which  con- 
cerns the  origin  of  the  germ-cells  themselves,  are  cyto- 
logical  in  their  terminology,  and  are  referable  to  the 
germplasm  (see  Figure  78),  as  contrasted  with  the 
Mendelian  and  biometric  aspects  of  genetics  which  are 
not  primarily  cytological  but  are,  on  the  contrary, 
statistical  in  method,  dealing  directly  with  somato- 
plasms. 

4.  THE  FACTORS  IN  SOMATOGENESIS 

Somatogenesis  deals  with  the  interaction  of  at  least 
two  sets  of  factors,  viz.,  (1)  hereditary,  and  (2)  en- 
vironmental. 


£54 


GENETICS 


Hereditary  factors  have  been  described  and  have  re- 
ceived the  major  share  of  attention  in  the  preceding 
pages.  Environmental  factors  may  upon  occasion, 


CYTOLOGICAL 

________________ 

Deals  with  the  origin  and  development  of  the  individual 
The  new  method  of  attacking  heredity 


ijnpv  /o  sdnoMJ  snojouinu  itfim,  8JD»Q 


IVDIASI.LVJ.S 
FIG.  78.  —  Any-side-up  Diagram  of  Genetical  Sciences. 


however,  cause  enormous  modifications  in  somatogenesis 
although  the  limits  of  variation  are  set  by  hereditary 
genes.  For  example,  genes  under  any  environmental 
circumstances  whatsoever  never  allow  an  egg  with  the 
heritage  of  a  worm  to  develop  into  a  bird,  nor  do  human 


SOMATOGENESIS  255 

genes  freighted  with  the  handicap  of  idiocy  ever  pro- 
duce an  intellectual  leader. 


5.  THE  ROLE  OF  GENES  IN  SOMATIC  DIFFERENTIATION 

An  essential  feature  of  cellular  differentiation  is  the 
unequal  division  of  material,  both  quantitatively  and 
qualitatively.  When  we  trace  the  complicated  adult 
organism  backward  step  by  step  to  the  fertilized  egg 
from  which  it  started  we  see  that  its  complexity  has 
arisen  largely  through  this  process  of  unequal  division. 

Moreover,  each  stage  in  the  "process  of  becoming"  is 
conditioned  upon  what  has  already  happened  in  pre- 
ceding stages,  since  differentiation  is  a  forward-moving 
sequence  of  events.  Just  as  the  roof  of  a  house  must 
follow  and  not  precede  the  erection  of  walls  which  are 
placed  on  a  foundation  previously  prepared,  so  the  he- 
reditary matter  in  the  gene  must  pass  through  a  long 
series  of  preliminary  steps  of  differentiation  before 
finally  coming  to  manifest  fruition  in  the  soma. 

Weismann,  who  by  the  process  of  logic  rather  than 
experimentally  located  the  germinal  substance  in  the 
nucleus  of  the  germ-cell,  assumed  an  elaborate  theoreti- 
cal system  of  "biophores,"  "ids,"  "idants,"  etc.,  where- 
by a  differential  distribution  of  the  nuclear  substance 
of  the  germ-cells  to  the  various  somatic  cells  is  supposed 
to  occur.  This  is  diagrammatically  shown  in  Fig- 
ure 79. 

Subsequent  discovery  and  confirmation  of  the  facts 
of  mitosis,  however,  have  shown  that  the  germplasm 
does  not  influence  the  development  in  this  way,  for 


£56 


GENETICS 


everything  indicates  that  the  entire  machinery  of 
mitosis  is  directed  toward  securing  an  equal  division 
of  the  heredity-determining  chromatin  to  the  two 
daughter-cells  at  each  division.  Ordinarily  the  entire 


SOMATOGENESIS  257 

chromatic  complex  is  handed  down  from  cell-generation 
to  cell-generation  in  the  development  of  the  soma  re- 
gardless of  the  type  of  tissue  to  be  formed.  The  ques- 
tion now  logically  follows : — How  can  identical  germinal 
substance  give  rise  to  different  products  in  different 
cells?  How  can  a  nerve  cell,  for  example,  so  depart 
from  its  embryonic  spherical  form  that  its  cytoplasm 
becomes  drawn  out  into  enormously  attenuated  neu- 
rones tingling  with  neuro-fibrils,  while  a  cartilage  cell, 
with  the  same  outfit  of  germinal  determiners  in  its 
nucleus,  commits  cytological  suicide  by  the  excessive 
secretion  of  its  cell  wall  ? 

DeVries  in  his  theory  of  "intra-cellular  pangenesis" 
(1889)  proposes,  as  a  way  out  of  this  dilemma,  enzy- 
matic "pangenes,"  of  which  each  nucleus  contains  a 
complete  set,  that  escape  into  the  cytoplasm  and  so 
control  its  differentiation, — an  explanation  "which 
nearly  meets  the  present  requirements  and  fits  pres- 
ent knowledge."  It  is  the  cytoplasm  and  not  the 
nucleus  that  differentiates,  although  the  directing 
stimulus  for  differentiation  comes  from  the  nucleus. 

This  conception  is  diagrammatically  shown  in  Fig- 
ure 80,  which  figure,  furthermore,  explains  how  the 
stamp  of  the  germplasm  upon  the  somatoplasm  can 
influence  not  only  immediate  cell-division  but  all  subse- 
quent ontogenetic  divisions  until  the  adult  structure 
results. 

6.  "CYTOPLASMIC  INHERITANCE" 

While  the  germinal  determiners  in  the  chromosomes 
are  being  apportioned  to  the  daughter  cells  in  mito- 
sis with  strict  impartiality,  the  cytoplasm  surrounding 


258 


GENETICS 


the  nucleus  does  not  meet  the  same  fate.  The  unequal 
distribution  of  the  cytoplasm,  even  in  the  early  cleavage 
stages  of  somatogenesis,  is  quite  apparent.  Moreover, 


SOMATOGENESIS  259 

in  the  cytoplasm  of  the  fertilized  egg  of  many  forms 
qualitative  differences  may  already  be  detected  that 
prophesy  clearly  the  course  which  differentiation  is 
to  take.  Conklin  cites  the  illuminating  case  of  the 
ascidian  Styela,  in  whose  egg  the  cytoplasm  in  differ- 
ent regions  varies  distinctively  in  color  so  that  these 
parts  may  be  unquestionably  followed  in  subsequent 
cleavage  and  their  fate  definitely  discovered. 

For  example,  the  peripheral  area  of  the  cytoplasm 
of  this  egg  containing  yellow  coloring  matter  finds  its 
way  into  the  cleavage  cells  which  become  muscles  and 
mesoderm;  a  gray  area  is  differentially  assorted  into 
cells  that  become  nervous  system  and  notochord;  a 
slate-blue  part  proves  to  be  the  source  of  epidermal 
cells  and  a  region  of  colorless  substance  gives  rise  to 
ectoderm  cells. 

Most  egg-cells  are  more  reticent  than  Styela  in 
revealing  the  part  that  their  cytoplasm  is  to  play  in 
ontogenesis,  but  it  has  been  possible  in  many  instances 
to  trace  cell-lineage  through  the  cleavage  stages  until 
the  results  of  differentiation  are  unmistakable  in  the 
tissues. 

The  fact  that  so  many  eggs  clearly  show  polarity 
and  indicate  the  future  symmetry  of  the  organism  be- 
fore development  has  begun  at  all  is  further  evidence  of 
the  important  part  that  the  cytoplasm  plays  in  soma- 
togenesis.  For  example,  when  the  fertilized  frog's  egg 
divides  for  the  first  time  into  the  two-cell  stage  these 
two  cells  are  the  ancestors  of  the  right  and  left  sides 
of  the  animal  and  the  cleavage  plane  between  them 
marks  the  future  long  axis  of  the  body. 


260  GENETICS 

Thus  while  the  chromosomes  with  their  invisible 
genes  are  the  ultimate  determiners  of  heredity,  the 
enveloping  cytoplasm  that  surrounds  the  nucleus,  par- 
ticularly of  the  egg-cell,  may  be  the  immediate  arbiter 
of  the  differentiation  processes  that  characterize  soma- 
togenesis.  "In  short,"  as  Conklin  says,  "the  egg  cyto- 
plasm determines  the  early  development  and  the  sperm 
and  egg  nuclei  control  only  the  later  differentiations. 
.  .  .  The  chromosomes  are  chiefly  concerned  in 
heredity,  the  cytoplasm  in  development." 

There  is  nothing  in  what  has  been  said  of  "cyto- 
plasmic  inheritance,"  however,  to  conflict  with  the 
generalization  that  the  real  determiners  of  heredity 
are  germinal,  for  it  is  the  genes  in  the  nucleus  of  the 
parent  germ-cell  that  gives  the  character  to  the  egg 
of  the  daughter-cell,  both  to  its  nucleus  and  to  its  cyto- 
plasm, although  the  latter  in  turn  influences  particu- 
larly the  early  stages  of  somatogenesis.  In  an  ex- 
cellent criticism  of  the  role  of  nucleus  and  cytoplasm 
as  vehicles  of  heredity,  Dunn  1  concludes : — "For  de- 
velopment, its  mechanism  is  but  grossly  known,  but  we 
have  learned  enough  of  the  determinative  effect  of  the 
nucleus  and  of  the  possibilities  of  interaction  betweei) 
cytoplasm  and  nucleus  to  foster  a  suspicion  that  one 
day  the  governance  of  the  chromosomes  over  develop- 
ment will  be  explained  in  physical  terms." 

7.  THE  PHYSICAL  STAGE-SETTING 

During  development  the  organism  is  beset  on  all  sides 
by  various  external  physical  factors,  which  are  more 
*Amer.  Nat.  vol.  LI,  1917,  p.  286. 


SOMATOGENESIS  261 

or  less  necessary  to  its  life,  and  the  modification  of  these 
factors  brings  about  a  corresponding  variation  in  the 
normal  progress  of  somatogenesis. 

These  external  factors,  such  as  temperature,  mois- 
ture, light,  chemical  solutions,  pressure,  etc.,  may  ac- 
celerate, retard  or  even  inhibit  the  normal  course  of 
events,  but  invariably  such  external  environmental  fac- 
tors contribute  largely  to  the  end  result  of  somatogene- 
sis. 

It  is  quite  likely  that  many  kinds  of  monsters  and 
defective  organisms  are  the  result  not  of  defective 
heredity  but  of  alterations  in  the  normal  constellation 
of  physical  factors  which  constitute  the  environment  of 
the  developing  organism. 

8.  THE  INTERNAL  ENVIRONMENT 

Not  only  is  somatogenesis  hedged  about  by  external 
modifying  factors  but  there  is  also  an  internal  environ- 
ment that  controls  to  a  large  degree  the  behavior  of 
hereditary  factors  and  determines  how  they  shall  come 
to  expression  in  the  somatoplasm. 

The  obvious  way  in  which  growth  is  dependent  upon 
the  intake  and  use  of  food,  and  the  abnormal  outcome 
following  an  unnatural  chemical  situation  within  the 
body,  such  as  the  presence  of  poisons,  are  illustrations 
of  what  is  meant  by  internal  environment.  Perhaps  the 
best  illustration  of  this  is  furnished  by  the  endocrine 
glands  in  mammals.  Twenty  years  ago  very  little  was 
known  with  certainty  about  the  part  that  these  ductless 
glands  play  in  the  organism,  but  they  have  become  so 


262  GENETICS 

important  in  modern  medical  research  that  endocrin- 
ology is  now  recognized  as  a  very  lusty  infant  in  the 
family  of  biological  sciences. 

The  chief  endocrine  structures  in  man  are  the  thy- 
roids, parathyroids,  the  two  functionally  and  anatomi- 
cally distinct  lobes  of  the  pituitary  gland,  pineal  gland, 
thymus,  adrenal  glands,  portions  of  the  pancreas 
and  the  various  sex  glands  (testes,  prostate,  ovaries, 
etc.).  These  structures  are  physiological  regulators 
and  have  to  do  with  the  growth  and  development  not 
only  of  the  body  but  also  of  the  mind.  Human  in- 
stincts, emotions,  mental  and  psychic  states  are  stimu- 
lated, inhibited,  altered  and  complicated  by  endocrine 
action.  The  endocrines,  therefore,  constitute  a  large 
part  of  the  machinery  through  which  heredity  must  act 
to  bring  about  its  results  and  consequently  it  is  possible 
to  control,  to  a  considerable  extent,  the  development 
and  behavior  of  man  through  the  internal  secretions 
produced  by  these  glands.  "Some  people  are  born  with 
so  stable  an  endocrine  relation,"  says  Bandler,  "that 
nothing  will  alter  the  normal  interaction  of  the  endo- 
crine glands ;  others  inherit  or  acquire  endocrines  so 
unstable  or  deficient  that  nothing  else  can  elevate  them 
to  the  threshold  of  the  normal." 

9.  THE  RATE  or  DEVELOPMENT 

No  doubt  one  essential  feature  in  the  development 
of  an  organism  is  differentiation  or  the  unequal  assort- 
ment of  material  as  already  mentioned  in  a  preceding 
paragraph,  but  another  factor  in  somatogenesis  is 


SOMATOGENESIS  263 

surely  the  time  element  as  it  appears  in  the  acceleration 
or  retardation  of  the  processes  concerned.  Not  all 
tissues  or  organs  develop  at  the  same  rate.  Some  out- 
run others  necessarily  in  order  to  prepare  the  way  for 
what  follows.  Under  normal  conditions  in  ontogene- 
sis things  swing  into  place  in  the  nick  of  time  to  make 
the  next  step  possible.  When  these  rhythms  are  upset, 
just  as  when  Field  Marshal  Grouchy  at  Waterloo 
failed  to  swing  his  troops  into  line  at  the  critical  mo- 
ment, then  there  results  a  Waterloo  in  the  organism. 
To  any  one  who  has  followed  in  detail  the  intricate 
stages  of  ontogenesis  in  some  organism,  conditioned  as 
it  is  by  its  indispensable  and  modifiable  environmental 
complex,  the  wonder  grows  that  the  successes  are  so 
many  and  the  disasters  so  few. 

10.  CONCLUSION 

It  is  not  enough  for  the  geneticist  to  know  the  chro- 
mosomal machinery  at  the  beginning  of  his  story  and 
the  Mendelian  moral  at  the  end  of  it.  Between  these 
two  fields  of  investigation  lies  the  no-man's  land  of 
somatogenesis  which  forms  an  important  part  of  the 
hereditary  tale. 

The  processes  of  somatic  differentiation  are  so  amen- 
able to  experimental  interference  that  no  doubt  future 
investigators  will  continue  to  be  attracted  to  the  cul- 
tivation of  this  promising  field  of  genetics. 


CHAPTER  XIII 

THE  DETERMINATION  OF  SEX 

1.  PREVALENT  IDEAS 

THE  mechanism  of  sex  determination  has  been  a 
matter  of  speculation  since  time  immemorial  and  many 
erroneous  as  well  as  impossible  ideas  remain  even  to-day 
in  the  mind  of  the  layman.  These  speculations  fall 
into  three  categories,  according  to  whether  the  belief 
is  held  (1)  that  the  sex  of  the  offspring  is  predeter- 
mined in  the  egg;  (2)  that  it  is  determined  at  the  time 
of  fertilization;  or  (3)  that  it  is  not  determined  until 
after  the  zygote  has  been  formed. 

All  the  older  experiments  on  sex  were  based  on  the 
last  of  these  suppositions.  It  was  believed  that  by 
varying  the  nutrition  of  the  developing  embryo  either 
sex,  as  desired,  could  be  obtained.  This  belief  was  ap- 
plied even  to  human  beings.  Experiments  on  tadpoles 
seemed  to  give  definite  positive  results,  but  we  now 
know  that  the  death  rate  in  these  experiments  was  so 
large  that  the  results  may  be  more  truly  explained  as 
due  to  differential  mortality. 

Others  held  that  the  age  or  vigor  of  the  parent  de- 
termines the  sex,  the  older  or  more  vigorous  of  the  two 
parents  tending  to  impress  its  sex  upon  the  offspring. 

Yet  another  belief,  and  one  still  held  by  many,  re- 
264 


THE  DETERMINATION  OF  SEX 

gards  the  freshness  or  staleness  of  the  egg  as  the  im- 
portant factor  in  predetermining  sex.  According  to 
this  idea  it  is  thought  that  an  egg  shortly  after  ovula- 
tion  tends  to  produce  a  female,  while  one  that  remains 
some  time  in  the  oviduct  tends  to  produce  a  male. 

The  idea  that  two  distinct  types  of  eggs  are  formed 
is  not  altogether  new.  Thus,  entirely  without  biological 
foundation,  the  theory  has  been  propounded  that  one 
ovary  gives  rise  to  male-producing  eggs  and  the  other 
forms  female-producing  eggs.  Equally  without  founda- 
tion is  the  theory  that  in  one  testis  male-determining 
spermatozoa  are  produced  and  in  the  other,  female- 
determining  spermatozoa. 

Modern  theories  of  sex  determination  hold  to  the 
first  and  second  of  the  three  possibilities  mentioned 
above.  If  there  are  two  kinds  of  eggs,  male-producing 
and  female-producing,  then  the  sex  of  the  individual  is 
already  fixed  at  the  time  of  the  extrusion  of  the  first 
polar  cell,  before  the  sperm-nucleus  has  united  with  the 
egg-nucleus  in  fertilization.  If  there  are  two  kinds  of 
sperm,  male-determining  and  female-determining,  then 
sex  depends  upon  the  type  of  sperm  uniting  with  the 
ovum,  and  it  may,  therefore,  be  said  that  sex  is  deter- 
mined at  the  time  of  fertilization. 

2.  SEX   CHROMOSOMES 

Our  present  day  stand  on  sex  determination  is  based 
entirely  on  direct  observation,  both  cytological  and  ex- 
perimental. In  1902,  an  unpaired  chromosome  was  ob- 
served by  McClung  in  the  testes  of  certain  Orthoptera. 


266  GENETICS 

This  he  called  a  sex-determiner.  The  association  of  this 
chromatic  body  with  sex  determination  proved  to  be 
a  discovery  of  primary  importance.  In  fact  it  opened 
a  new  era  in  cytology  and  heralded  the  beginning  of 
a  large  number  of  experiments  and  much  profitable  dis- 
cussion dealing  with  the  mechanism  of  sex  determina- 
tion. 

In  many  groups  of  animals  there  is  an  unpaired 
chromosome  in  the  male,  called  the  ^-chromosome, 
which  may  be  seen  in  the  somatic  cells,  in  the  sperma- 
togonia  and  in  the  spermatocytes  (Fig.  81).  In  the 
female  cells,  both  somatic  and  germinal,  the  #-chromo- 
some  is  paired.  During  the  process  of  spermatogenesis 
the  autosomes,  that  is,  the  remaining  chromosomes 
which  have  nothing  to  do  with  the  determination  of  sex 
pair  to  form  tetrads,  in  the  first  spermatocyte  division, 
but  are  later  reduced  to  dyads,  in  the  second  sperma- 
tocyte division,  when  the  ^-chromosome  passes  un- 
divided to  one  of  the  second  series  of  spermatocytes 
(Fig.  81).  The  other  spermatocyte  of  the  second 
division  accordingly  receives  no  part  of  the  sex-deter- 
mining material.  During  the  division  into  spermatids 
the  former  second  spermatocyte,  now  freighted  with 
the  .r-chromosome,  gives  rise  to  a?-bearing  cells  which 
form  the  female-determining  sperm,  while  the  other 
second  spermatocyte,  which  did  not  receive  an  iT-chro- 
mosome,  gives  rise  to  two  male-determining  spermato- 
zoa. 

Hence,  any  zygote  receiving  two  sets  of  autosomes 
and  two  ^-chromosomes  becomes  a  female,  while  a 
zygote  receiving  two  sets  of  autosomes  and  only  one 


THE  DETERMINATION  OF  SEX        267 


^-chromosome  becomes  a  male.     Since  both  types  of 
sperm   are  ordinarily   formed   in   equal  numbers,   the 


FEMALE 


Secondary 
Oocyte 


Matured 
Ovum 


FIG.  81. — Sex  determination  in  the  case  of  heterogametic  males. 

chances  that  a  male-  or  a  female-determining  sperm 
will  reach  the  egg  in  the  process  of  fertilization,  are 


268 


GENETICS 


equal  and  the  resulting  zygotes,  therefore,  are  approxi- 
mately 50  per  cent  male  and  50  per  cent  female 
(Fig.  82). 

A.   THE  Y-CHROMOSOME 

The  foregoing  is  the  simplest  case  of  sex-determina- 
tion known  and,  while  this  is  the  fundamental  type, 
still  there  are  many  variations  of  the  mechanism.  For 
example,  the  ^-chromosome  may  have  a  "y"  partner 
in  the  male  cells,  in  which  case,  if  n  =  the  haploid,  or 


FIG.  82. — Diagram  to  show  how  numerical  equality  of  the  sexes 
results  when  one  parent  is  homozygous  (the  female  in  this 
instance)  and  the  other  is  heterozygous  for  the  sex  character. 

halved,  set  of  autosomes  in  a  given  animal,  then  the 
following  formula  holds  true: — 

2n  -f-  xy  =  male  and  2n  +  xx  =  female. 

In  the  spermatogonia  of  animals  maturating  in  this 
manner,  half  the  spermatids  receive  an  ^-chromosome 
and  half  a  ^-chromosome,  the  latter  being  the  male- 
determining  spermatozoa. 

In  certain  other  cases  the  ^-chromosome  may  be 
represented  by  several  discrete,  i.e.,  separate,  compo- 
nents, and  it  may  or  may  not  have  a  ^-chromosome 


THE  DETERMINATION  OF  SEX        269 

associated  with  it  in  the  male  cells.  Thus,  in  Gelas- 
tocoris,  a  hemipteron,  the  male  is  represented  by  the 
formula  2n  +  kx  +  y  and  the  female  by  2n  -f-  &r. 
Here  "n"  equals  fifteen,  so  that  the  male  diploid  num- 
ber of  chromosomes  is  thirty-five  and  the  female, 
thirty-eight. 

Until  recently  the  ^-chromosome  has  not  been  known 
to  carry  specific  genes  for  bodily  characters.  Indeed, 
this  chromosome  has  been  generally  regarded  as  merely 
a  degenerate  ^-chromosome  that  has  lost  its  sex  genes 
and  most  of  its  other  genes  as  well.  That  it  is 
essential  to  the  typical  development  of  those  species 
where  it  is  normally  present  has  been  proven  in  the 
non-disjunction  experiments  of  Bridges.  A  male 
Drosophila  without  the  ^-chromosome,  for  instance,  is 
sterile. 

In  many  forms  it  is  not  unlikely  that  there  is  no  sex- 
determining  mechanism  visible  even  with  the  aid  of  the 
best  microscopes,  but,  nevertheless,  it  is  probable  that 
x-  and  ^/-chromosomes  exist,  and  that  the  ^-chromo- 
somes are  practically  equal  to  the  ^-chromosomes  in 
size,  differing  from  them  merely  in  the  absence  of 
specific  genes. 

B.    SEX    GENES 

In  the  female,  except  in  those  cases  where  difference 
in  chromosomal  size  is  present,  the  ^-chromosomes  can- 
not always  be  distinguished  from  ordinary  autosomes 
and  it  is  furthermore  known  from  breeding  experiments 
that  they  bear  many  genes  for  characters  having  noth- 
ing to  do  with  sex. 


270  GENETICS 

That  there  are  specific  genes  in  the  z/-chromosome 
which,  working  in  conjunction  with  autosomal  genes 
are  capable  of  producing  males,  females  or  interme- 
diates, in  cases  where  the  normal  relationship  is  upset, 
has  been  indicated  very  clearly,  especially  by  Bridges  in 
recent  experiments  on  DrosophUa. 

Sex,  in  other  words,  is  now  put  upon  a  basis  of 
specific  genes.  We  are,  therefore,  entirely  rid  of  the 
older  ideas  that  the  tf-chromosome  is  composed  of  a 
different  kind  of  chromatin  from  that  found  in  the 
autosomes  and  that  the  sex  of  the  zygote  depends  upon 
the  amownt  of  a?-chromatin  it  receives. 


C.    HETEROGAMETIC    FEMALES 

The  reverse  of  the  foregoing  mechanism,  in  which 
two  kinds  of  sex-determining  sperm  are  present,  is 
found  in  the  Lepidoptera  and  birds.  In  these  groups 
the  presence  of  2n  -f-  xx  constitutes  a  male  and  2n  -\-x, 
a  female.  The  formulae  in  these  cases  are  usually  writ- 
ten 2n  -f-  zz  and  2n  +  2,  in  order  to  distinguish  them 
from  those  of  heterogametic  males. 

The  cytological  proof  for  the  s-chromosomes  is  not 
as  strong  as  for  the  o?-chromosomes,  since  both  avian 
and  lepidopteran  chromosomes  are  peculiarly  difficult 
to  study.  Nevertheless,  the  facts  are  well  borne  out  by 
breeding  experiments  in  both  groups. 

Definite  results  have  been  reached  by  Seiler  and  also 
by  Doncaster  in  experiments  with  moths,  showing  that 
two  types  of  ova  are  produced,  namely,  one  which, 
after  extruding  the  ^-chromosome  into  the  polar  cell 


THE  DETERMINATION  OF  SEX        271 
MALE  FEMALE 


% 


Mature  Ova 
FIG.  83. — Sex  determination  in  the  case  of  heterogametic  females. 

and  becoming  fertilized,  produces  females,  and  another 
which,  retaining  the  ^-chromosome,  produces  males. 
It  is  obvious  that  in  this  case  (see  Fig.  83),  the  sex 


272  GENETICS 

of  the  zygote  depends  entirely  upon  the  method  of 
maturation  of  the  ovum,  the  retention  or  expulsion  of 
the  ^-chromosome  being  the  deciding  factor  in  the  de- 
termination of  sex.  If  in  any  way  maturation  can  be 
controlled  by  factors  exerting  an  influence  either  from 
within  the  egg  itself  or  external  to  it,  then  sex  ratios 
may  be  altered  from  the  normal  50  : 50.  This  has 
been  done  by  Seller  in  the  case  of  moths  by  controlling 
the  temperature  of  the  developing  ova  at  the  critical 
time  in  the  process  of  maturation. 

The  control  of  maturation  offers  a  possible  explana- 
tion of  such  sex  ratios  as  have  been  obtained  by  Riddle 
in  his  forced  breeding  experiments  on  doves,  where 
females  are  produced  in  the  latter  part  of  the  breeding 
season  from  large  eggs  and  males  in  the  early  part 
of  the  season  from  small  eggs. 

3.  SEXUAL  CYCLES 
A.  APHIDS  AND  PHYLLOXEBANS 

Most  enlightening  observations  on  the  determina- 
tion of  sex  by  means  of  influencing  maturation,  have 
been  made  upon  aphids  and  phylloxerans  by  Morgan 
and  by  Von  Baehr.  It  is  well  known  that  in  the  case 
of  Aphis  fertilized  eggs  always  produce  females.  Under 
favorable  conditions  both  males  and  females  are  pro- 
duced parthenogenetically,  the  males,  however,  always 
arising  from  smaller  eggs  than  the  females. 

It  has  been  observed  too  that  in  these  smaller  eggs 
(Fig.  84)  an  entire  ^-chromosome  is  extruded  in  the 
giving  off  of  the  one  polar  cell,  leaving  in  the  egg 


THE  DETERMINATION  OF  SEX        273 
APHID-PHYLLOXERAN  CHROMOSOME  CYCLE 


Soma  of  stem-mother  and 
migrantfemales  (her  daughters) 


Parthenogenetic  eggs 
of  migran  t  females 

giving  rise          ^ — v  * 
to  JT          ^^ 


Anaphase  of  the  single 
maturation  division 


Fio.  84. — The  chromosome  cycle  in  parthenogenesis  of  aphids  and 
phylloxerans. 

2n  +  #  chromosomes  (five  in  number)  and  that  such 
an  egg  forms  a  male.    On  the  other  hand,  in  the  larger 


274  GENETICS 

parthenogenic  eggs  no  whole  ^-chromosome  is  extruded 
into  the  single  polar  cell  given  off  and  consequently  the 
egg,  retaining  2n  -\-  xx  chromosomes  ( six  in  number) , 
develops  into  a  female. 

In  the  spermatogenesis  of  these  forms  it  has  been 
found  that  only  one  secondary  spermatocyte  develops 
from  each  primary  spermatocyte,  namely,  the  one 
which  receives  the  J7-chromosome.  Thus,  only  two  in- 
stead of  four  spermatids  result  from  a  primary  sper- 
matocyte and  these  two  form  female-determining  sper- 
matozoa. The  "winter  eggs"  of  these  insects  have  two 
maturation  divisions  reducing  the  chromosomes  to  the 
haploid  condition.  The  female  diploid  number  is  re- 
stored upon  fertilization. 

It  would  seem,  therefore,  that  in  the  phylloxerans 
and  aphids  at  least,  maturation  is  actually  controlled 
by  the  size  and  composition  of  the  egg. 

B.   ROTIFERS  AND   DAPHNIDS 

It  is  unfortunate  that  the  rotifers  and  daphnids, 
which  lend  themselves  so  favorably  to  breeding  experi- 
ments, are  not  as  favorable  cytological  material  as  the 
homopterons,  for  it  is  not  at  all  unlikely  that  their 
sex-determination  rests  upon  a  similar  basis  to  that 
above  described. 

In  rotifers  and  daphnids,  as  in  homopterons,  fer- 
tilized eggs  give  rise  to  females,  whereas  during  par- 
thenogenesis both  females  and  males  may  arise,  the 
latter  coming  from  smaller  eggs  than  the  former. 
These  facts  are  all  the  more  interesting  for  the  reason 


THE  DETERMINATION  OF  SEX        275 

that  Whitney  and  A.  F.  Shull,  each  working  separately 
on  rotifers,  have  been  able,  through  modification  of  ex- 
ternal conditions,  to  alter  the  normal  cycle  of  repro- 
duction, by  causing  the  continuance  of  the  partheno- 
genetic  process  beyond  the  normal  limit. 

It  seems  evident  that,  through  the  modification  of 
external  conditions,  they  have  succeeded  in  influencing 
the  type  of  egg  produced.  If  this  case  is  really  paral- 
lel to  that  of  Aphis  and  Phylloxera,  then  the  type  of 
egg  artificially  produced  ought  thereafter  to  control 
its  own  maturation. 

In  daphnids,  where  parthenogenesis  alternates  with 
the  sexual  cycle,  at  least  three  kinds  of  eggs  are  pro- 
duced; (1)  thick-shelled,  fat-laden,  ephippial  eggs 
which  must  be  fertilized  in  order  to  develop;  (2)  thin- 
shelled,  glycogen-laden,  parthenogenetic  eggs,  which 
develop  into  females  without  fertilization;  and  (3) 
thin-shelled,  smaller,  parthenogenetic  eggs  which  de- 
velop into  males.  The  type  of  egg  produced,  as 
shown  by  Smith,  may  be  influenced  by  temperature  and 
also  by  food.  It  is  not  improbable  that  we  may  yet 
discover  in  the  maturation  of  these  ova  differences  in 
chromosomal  behavior  correlated  with  each  type  of 
ovum  and  the  sex  of  the  resulting  offspring. 

C.    THE    HONEY    BEE 

Closely  allied  to  the  problem  of  the  sex  cycle,  as 
described  in  experiments  with  the  homopterons,  is  the 
question  of  sex-determination  as  observed  in  the 
hym^noptera. 


276  GENETICS 

Even  before  chromosomes  were  known,  Dzierzon 
postulated  that  males  of  this  group  (drones)  are 
formed  from  unfertilized  eggs,  and  females  (worker 
and  queens)  from  fertilized  eggs,  a  view  which  has 
been  substantiated  by  both  cytological  and  genetical 
observations.  Newell  has  shown  that  in  the  cross  be- 
tween Italian  (gray)  queens  and  German  (dark) 
drones,  as  well  as  in  reciprocal  crosses,  the  male  off- 
spring are  purely  maternal,  while  the  females  are 
hybrid  in  character.  Cytological  observations  by 
Petrunkevitch  and  by  Nachtsheim  have  also  estab- 
lished the  validity  of  the  Dzierzon  theory. 

Coupled  with  this,  studies  on  the  spermatogenesis 
of  hymenoptera  have  revealed  the  fact  that  the  sper- 
matogonia  possess  solely  the  haploid  number  of  chro- 
mosomes, and  in  order,  therefore,  that  this  number  be 
not  further  reduced  in  the  process  of  maturation,  only 
one  division  of  chromatin  takes  place.  In  the  first 
spermatocyte  division  of  the  honey  bee  all  the  chro- 
matin passes  to  a  single  chromosome,  only  a  minute 
degenerate  non-chromatic  globule  being  formed  at  the 
other  pole  of  the  spindle.  In  the  second  spermatocyte 
division  the  chromatin  divides  but  one  of  the  sper- 
matids  is  very  small  and  degenerates.  Thus,  instead 
of  four  spermatids,  only  one  is  formed  and  this  one 
contains  the  haploid  number  of  chromosomes. 

Variations  of  this  process  are  found  in  other 
hymenoptera  which  frequently  result,  in  the  formation 
from  the  larger  second  spermatocyte,  of  two  separate 
spermatids  each  possessing  the  haploid  number  of 
chromosomes. 


THE  DETERMINATION  OF  SEX        277 

4.    P<XLYEMBRYONY 

Closely  allied  to  the  chromosomal  basis  of  sex  are 
the  facts  of  polyembryony,  for  when  more  individuals 
than  one  are  formed  from  a  single  ovum  they  are  inva- 
riably of  the  same  sex.  Classical  examples  are  para- 
sitic hymenoptera,  principally  of  the  families  Procto- 
trypidoe  and  Chalcididce,  in  which  thousands  of  indi- 
viduals often  result  from  a  single  egg.  Other  examples 
are  the  quadruplets  formed  in  the  nine-banded  arma- 
dillo, Tatusia,  and  identical  or  monochorial  twins  in 
man  and  other  mammals. 

In  the  case  of  mammals  the  type  of  sperm,  either 
with  or  without  the  ^-chromosome,  is  undoubtedly  the 
deciding  factor  in  sex  determination,  for  the  reason 
that  when  all  of  the  chromosomes  of  the  zygote  divide 
normally  the  sex  of  the  resulting  individuals  must  be 
the  same.  In  other  ways  also  they  will  be  genetically 
identical. 

In  hymenoptera  sex  depends  entirely  upon  whether 
fertilization  or  parthenogenesis  takes  place.  A  fer- 
tilized egg  will  result  in  females  and  an  unfertilized  one 
in  males,  a  supposition  based  upon  direct  cytological 
observation.  The  facts  of  polyembryony  thus  offer 
strong  substantiation  to  the  idea  of  chromosomal  de- 
termination of  sex. 

5.  SEX-LINKED  INHERITANCE 

The  association  of  Mendelian  characters  with  par- 
ticular chromosomes  is  nowhere  better  shown  than  in 


278  GENETICS 

the  case  of  sex-linked  characters,  the  genes  for  which 
are  undoubtedly  located  in  the  sex-chromosomes,  and 
whose  inheritance  follows  exactly  the  distribution  of 
these  chromosomes.  About  thirty  genes  of  this  kind 
have  been  discovered  in  Drosophila  alone.  (See  Fig. 
76,  the  left  hand  line.) 

Sex-linked  inheritance,  which  means  that  genes  for 
characters  other  than  sex  are  associated  with  a  par- 
ticular sex,  i.e.,  are  carried  in  the  same  chromosome 
that  bears  the  sex-determining  genes,  should  not  be 
confused  with  sex-limited  characters,  i.e.,  with  secon- 
dary sexual  characters  that  are  found  in  one  sex  only 
but  the  genes  for  which  may  be  located  in  any  chro- 
mosome. 


An  example  of  a  dominant  sex-linked  character  is 
the  red  eye  of  Drosophila.  The  manner  of  its  inherit- 
ance is  as  follows. 

If  a  red-eyed  female  is  mated  with  a  white-eyed 
male  (Fig.  85),  the  Fj  generation  are  all  red-eyed, 
and  when  members  of  the  F1  generation  are  inbred  the 
F2  generation  shows  the  expected  proportion  of  three 
red-eyed  individuals  to  one  white-eyed.  However,  a 
peculiar  result  appears  inasmuch  as  all  of  the  white- 
eyed  individuals  are  males.  Thus,  one  half  of  the  F2 
males  are  white-eyed  like  their  grandfathers  while  all 
of  the  Fj  females  are  red-eyed  because  the  character 
of  white-eyes  is  covered  up  when  the  gene  for  red  is 
present.  The  eggs  of  FA  females,  however,  which 
eliminate  the  genes  for  red  eyes  in  the  polar  body 


THE  DETERMINATION  OF  SEX        279 

during  maturation  and  are  then  fertilized  by  a  sperm 
bearing  a  ^-chromosome,  mature  into  white-eyed  off- 
spring. 

The  reciprocal  cross  of  white-eyed  females  with  red- 

Red-eyed  Female  White-eyed  Male 


Parental 

XX 

XT 

,    .y 

Gametes 

'* 

XX 

•T  fp>  

XY    ^ 
jr 

Gametes 


Tjt  "V    "V  V    V  "V   V  V    XT 

x"o  _^-, _•"•.  •**•    •**•  •**•_   -^  -t»-    .* 

FIG.  85.— Criss-cross  inheritance.  The  underscored  X  means  the 
presence  of  the  genes  for  red  eyes  in  the  sex-chromosome. 
The  male  is  heterozygous. 

eyed  males,  gives  an  entirely  different  result  (Fig. 
86.)  It  will  be  seen  that  in  this  case  the  Ft  females 
are  red-eyed  like  their  fathers,  while  the  males  are 
white-eyed  like  their  mothers.  In  the  F2  generation 
half  of  the  males  and  half  of  the  females  are  white- 
eyed  and  the  others  are  red-eyed,  due  to  the  fact  that 
the  male  mechanism  which  has  only  one  ^-chromosome, 


280 


GENETICS 


is  capable  of  bearing  the  gene  for  red  in  only  half  of 
its  germ-cells.  The  F±  females,  which  normally  carry 
two  ^-chromosomes,  all  receive  an  tf-chromosome  from 
their  father  and  are  consequently  red-eyed,  while  the 

White-eyed  Female  Red-eyed 


XX 


XY 


Gametes 


Gametes 


XX 


XX 


XY 


XY 


FIG.  86. — Criss-cross  inheritance.  The  reciprocal  cross  to  that 
shown  in  Fig.  85.  All  individuals  with  underscored  X  have 
red  eyes.  The  male  is  heterozygous. 

Fj  males  all  receive  a  single  ^-chromosome  from  their 
white-eyed  mother  and  are,  therefore,  themselves  white- 
eyed. 

B.    COLOR-BLINDNESS    IN    MAN 

This  criss-cross  type  of  inheritance  has  long  been 
known  in  man,  color-blindness  being  perhaps  the  best 


THE  DETERMINATION  OF  SEX        281 

known  example  of  a  sex-linked  character,  behaving  in 
its  inheritance  exactly  as  that  of  red-eye  in  Drosophila. 
That  color-blind  females  are  so  rare  is  due  to  the  fact 
that  it  requires  a  duplex,  or  homozygous,  dose  of  the 
determiner  for  color-blindness  to  produce  a  color-blind 
female,  while  only  a  simplex,  or  heterozygous,  dose  is 


Gametes 


Gametes 


FIG.  87. — General  diagram  for  sex-linked  inheritance.  The  under- 
scored symbol  (X)  represents  a  sex  determiner  with  some 
other  character  (as  color-blindness)  linked  with  it. 


needed  to  produce  a  color-blind  male.  These  facts 
agree  perfectly  with  the  idea  that  the  female  is  homo- 
zygous and  the  male  heterozygous  with  respect  to  sex, 
and  that  the  factor  for  color-blindness  is  linked  with 
the  determiner  for  sex.  Sex-linked  inheritance,  as 
shown  in  this  case,  may  be  illustrated  by  the  diagram 
above  (Fig.  87)  in  which,  for  the  sake  of  simplicity, 
only  sex  chromosomes  and  the  determiners  for  color- 


282 


GENETICS 


blindness  are  represented.     Underscored  X  represents 
a  color-blind  determiner  linked  to  a  sex  chromosome. 

From  this  diagram,  which  agrees  substantially  with 
the  facts,  it  is  apparent  that  a  color-blind  male  mated 
to  a  normal  female  will  produce  no  color-blind  off- 
spring, although  the  females  will  be  "carriers"  of 
color-blindness,  that  is,  will  possess  the  factor  in  sim- 
plex form  and  will,  therefore,  carry  it  for  the  female 
in  a  latent  condition. 


PARENTS 

EXPECTED  OFFSPRING 

$ 
Normal 

9 
Color-blind 

8 
Color-blind 

$ 
Carrier 

Normal  T 

Carrier 

£  color-blind 
%  normal 

\  carrier 
£  normal 

Color-blind 

Normal 

Normal 

Carrier  . 

Color-blind 

Color-blind 

Color-blind 

Color-blind 

Color-blind 

Carrier 

$  color-blind 
\  normal 

\  color-blind 
\  carrier 

The  sons  of  such  a  mating  having  a  normal  mother 
and  a  color-blind  father  will  be  absolutely  free  from 
the  defect  and  cannot  produce  color-blindness  in  any 
of  their  offspring  when  mated  with  a  normal  strain. 
If,  however,  the  "carrier"  daughters  from  such  a 
parentage,  who  are  genotypically  heterozygous  for 
color-blindness  but  phenotypically  normal,  mate  with 
normal  individuals,  the  expectation  is  that  one  half 
of  the  sons,  and  none  of  the  daughters  will  be  color- 
blind, but  that  one  half  of  these  daughters  will  carry 


THE  DETERMINATION  OF  SEX        283 

the  color-blind  determiner  in  simplex  form,  that  is,  in 
a  condition  ineffective  for  producing  color-blindness 
in  female  individuals. 

All  of  the  various  possibilities  in  the  inheritance  of 


Barred  Male 


Female 


Z  0 


Gametes 


Gametes 


Z  Z 


Z  O 


FIG.   88. — Sex-linked  inheritance,   with   the   female   heterozygous. 
The  "barred"  character  is  indicated  by  underscored  letters. 

color-blindness   according  to   the   sex-linked   interpreta- 
tion are  indicated  in  the  table  on  page  282. 


C.  THE  BARBED  PLYMOUTH  BOCK 

In   animals    in  which   the   female  is   heterogametic 
(Lepidoptera  and  birds)  sex-linked  characters  are  like- 


284 


GENETICS 


wise  known  to  exist  and,  in  fact,  were  first  discovered 
in  moths  by  Doncaster.  In  these  cases  it  is  the  fe- 
male instead  of  the  male  that  possesses  the  mechanism 
whereby  the  character  in  question  can  be  present  only 
once. 

Black  Male  Barred  Female 


Gametes 


Z  Z 

z — 


zo 


Gametes 


Z  O 


z  z 


z  o 


FIG.  89. — Sex-linked  inheritance,  with  the  female  heterozygous. 
Reciprocal  cross  to  that  shown  in  Fig.  88.  The  "barred"  char- 
acter is  indicated  by  the  underscored  gametes. 

For  example,  "barring"  is  a  dominant  sex-linked 
trait  in  poultry,  as  shown  in  Figures  88  and  89. 

In  the  cross  shown  in  Figure  88  all  the  males  and  half 
the  females  in  the  F2  generation  are  barred  while  in 
the  reciprocal  cross  shown  in  Figure  89  the  Fj  males  are 
barred  because  they  have  a  ^-chromosome  from  their 


THE  DETERMINATION  OF  SEX        285 

maternal  side,  while  the  Fx  females  are  black  because 
their  single  s-chromosomes  came  from  their  black 
father. 

6.  NON-DISJUNCTION 

A  striking  confirmation  of  the  chromosomal  inter- 
pretation of  sex  is  furnished  by  the  phenomenon  of 
non-disjunction  discovered  in  1913  by  Bridges.  In 
attempting  to  explain  certain  unexpected  ratios  which 
he  obtained  in  a  long  series  of  breeding  experiments 
upon  white-eyed  Drosophilas,  Bridges  found  that  his 
results  would  be  more  intelligible  if  what  he  termed 
"non-disjunction"  was  assumed  to  occur. 

By  non-disjunction  is  meant  that  both  the  #-chro- 
mosomes  instead  of  disjoining  and  going  normally  to 
the  two  poles  during  the  last  maturation  division, 
remain  attached  to  each  other  and  pass  together  to 
one  pole  leaving  the  other  pole  without  any  #-chromo- 
some.  In  consequence,  half  the  mature  eggs  should  be 
provided  with  two  d?-chromosomes  and  half  with  none 
at  all.  Cytological  examination  of  these  unusual  flies 
showed  that  this  was  what  actually  did  sometimes 
happen. 

The  progeny  of  non-disjunctional  white-eyed  females, 
as  shown  in  Figure  90  taken  from  Sharp's  "Introduc- 
tion to  Cytology",  show  a  theoretical  diversity  of 
characters  which  is  borne  out  in  the  results  of  actual 
breeding.  Morgan  sums  the  matter  up  when  he  says : — 
"An  abnormal  distribution  of  sex-chromosomes  goes 
hand  in  hand  with  an  abnormal  distribution  of  all  sex- 
linked  factors." 


286 


GENETICS 


THE  DETERMINATION  OF  SEX        287 

Explanation  to  Figure  90 

"Non-disjunction  and  its  results  in  Drosophila.  The  two  large 
circles  in  the  first  row  represent  male  and  female  flies  producing 
sperms  and  eggs  respectively.  Non-disjunction  in  the  female  gives 
2  kinds  of  eggs,  with  XX-  and  no  sex-chromosomes,  instead  of 
the  normal  single  kind  with  one  X.  At  fertilization  there  are 
possible  4  combinations  rather  than  2,  as  shown  in  the  large  circles 
of  the  second  row.  Owing  to  the  several  ways  in  which  her  3  sex- 
chromosomes  may  be  distributed  at  maturation,  the  female  repre- 
sented by  the  third  circle  produces  4  kinds  of  eggs.  When  mated 
to  a  normal  male  (below  the  horizontal  line)  with  two  kinds  of 
sperms,  8  combinations  are  possible  (last  row).  Numbers  1,  4 
and  5  are  normal  flies  and  give  the  usual  type  of  progeny.  Num- 
bers 2,  6  and  7,  owing  to  the  presence  of  3  sex-chromosomes,  give 
exceptional  results  when  bred.  Types  Numbers  3  and  8  do  not 
appear  in  the  cultures,  probably  because  they  die  very  early. 
The  original  male  has  red  eyes  and  the  original  female  white 
eyes.  Red  eyes  (represented  by  the  dots)  appear  in  every  fly 
bearing  the  X-chromosome  of  the  original  male." 

(Diagram  by  Sharp  based  on  data  from  Bridges  and  Morgan.) 

7.  SECONDARY  SEXUAL  CHARACTERS  AND  HORMONES 

It  will  be  seen  from  the  preceding  illustrations  that 
the  primary  differences  between  the  sexes  is  in  the  kind 
of  gametes  which  they  form.  The  female  is  an  egg- 
producer,  the  male  a  sperm-producer.  In  many  ani- 
mals especially  invertebrates,  it  is  very  difficult  to  dis- 
tinguish males  from  females  without  first  examining 
the  gonads,  although  there  is  no  lack  of  forms  in  which 
one  can  with  ease  distinguish  the  sexes  solely  by  ex- 
ternal appearances. 

Very  often  this  sexual  dimorphism  is  confined,  first, 
to  the  genitalia  or  to  accessory  apparatus  used  in 
copulation,  oviposition,  or  rearing  of  the  young;  and 
second,  to  extra  genital  characteristics  not  associated 
directly  with  reproduction,  such  as  color,  ornamenta- 
tion, and  the  like.  Both  of  these  types  of  sexual 


288  GENETICS 

dimorphism  are,  however,  secondary  to  gamete  pro- 
duction. 

In  mammals  and  birds  these  so-called  secondary 
sexual  characters  are  found  to  be  largely  dependent 
for  their  proper  development  upon  the  normal  presence 
and  activity  of  the  gonads.  For  example,  castration 
of  young  male  mammals  results  in  individuals  lacking 
in  many  ways  the  attributes  of  normal  males.*^Among 
cattle  and  horses,  which  have  undergone  this  opera- 
tion, the  fiery  males  become  docile  and  lack  the  thick 
neck  common  to  their  kind.  They  also  put  on  fat  more 
readily.  In  man  the  voice  fails  to  change,  the  beard 
is  weak,  the  epiphyses  of  the  bones  do  not  fuse  and  the 
spirit  is  dulled.  Females  deprived  of  ovaries  early  in 
life  fail  to  develop  normal  mammary  glands,  while 
certain  of  their  skeletal  characters  are  likewise  much 
altered.  Extensive  experiments  have  proved  that  in 
birds  and  mammals  secretions  of  the  gonads,  known 
as  hormones,  are  essential  to  normal  development.  The 
castration  of  young  male  rats  followed  by  ingraft- 
ing of  ovaries  causes  these  individuals  to  become  femin- 
ized in  character. 

Perhaps  no  better  case  of  the  influence  of  hormones 
is  known  than  that  of  the  "free  martin,"  adequately 
explained  by  the  observations  of  Lillie.  He  found  in 
cattle  that  when  the  chorionic  coverings  of  twin  em- 
bryos of  opposite  sex  fuse  so  that  the  blood  vessels 
anastomose,  the  more  rapidly  developing  male  embryo 
sends  out  hormones  into  the  circulation  which  inhibit 
the  normal  development  of  the  female  embryo.  The 
much  modified  female  embryo  may  then  be  born  as  a 


THE  DETERMINATION  OF  SEX        289 

free  martin  in  which  the  ovaries  tend  to  form  tubules 
quite  like  those  of  a  testis. 

In  birds  the  activities  of  the  gonads  likewise  control 
to  a  large  extent  the  development  of  the  secondary 
sexual  characters,  as  has  well  been  shown  by  Goodale 
and  by  Morgan  in  castration  and  transplantation  ex- 
periments on  ducks  and  fowls.  Most  striking  is  the 
case  of  female  birds  which,  when  castrated  while  still 
young,  develop  male  plumage  and  posture. 

It  has  been  clearly  demonstrated  that  the  genes  for 
secondary  sexual  characters  lie  in  the  autosomes  and 
thus  both  male  and  female  have  determiners  for  the 
secondary  sexual  characters  of  both  sexes.  For  ex- 
ample, normally  in  cases  where  the  male  is  heteroga- 
metic,  the  presence  of  a  single  ^-chromosome  in  all  of 
its  cells,  together  with  the  endocrine  secretion  of  its 
gonads,  causes  the  male  genes  for  secondary  sexual 
characters  to  develop  and  those  of  the  female  to  be 
suppressed.  By  castration  and  transplantation  the 
normal  condition  may  be  upset  and  the  female  secon- 
dary sex  genes  brought  into  action. 

The  whole  problem  of  sex-hormones  is  very  compli- 
cated since  it  has  been  shown  that  the  secretion  from 
the  gonads  is  merely  one  link  in  the  chain  of  endocrine 
factors  which  tend  to  set  into  action  the  genes  for 
determining  secondary  sexual  characters.  In  the  de- 
velopment of  sex  in  the  vertebrates  the  genes  for  the 
production  of  these  sex-hormones  are  second  in  impor- 
tance only  to  those  genes  which  determine  whether  ova 
or  sperm  shall  be  formed  in  an  individual. 


290  GENETICS 

8.  THE  EFFECT  OF  PARASITISM  ON  SEX 

It  has  been  well  demonstrated  in  insects  that  castra- 
tion, even  of  very  young  individuals,  produces  no  effect 
upon  the  secondary  sexual  characters  when  the  animal 
reaches  its  adult  form.  Even  the  implantation  of 
gonads  of  the  opposite  sex  results  in  no  change.  The 
growth  and  development  of  the  soma  seems  to  be  fixed 
by  the  chromosomal  complex  and  does  not  appear  to 
be  influenced  by  the  action  of  any  sex-hormone.  Altera- 
tions of  secondary  sexual  characters  may  occur,  how- 
ever, by  means  of  parasitism,  as  shown  by  experiments 
on  Crustacea  and  insects. 

Among  Crustacea  the  best  case  of  this  kind  per- 
haps is  that  of  the  crab  Inachus,  the  male  of  which 
when  parasitized  by  the  cirripede  Sacculina,  as  de- 
scribed by  Smith,  becomes  similar  to  the  normal  female 
in  the  form  of  its  claw,  abdomen  and  abdominal  ap- 
pendages. 

Among  insects  Thelia  bimaculata,  described  by  Korn- 
hauser,  is  a  good  example.  Parasitized  males  resemble 
females  even  to  the  minute  structure  of  their  chitinous 
integument.  Such  alterations  are  due,  very  likely,  to 
an  entire  upset  in  the  metabolism  of  the  host,  changing 
the  internal  environment  so  fundamentally  that  the 
genes  for  the  male  secondary  sexual  characters  fail  to 
find  the  conditions  necessary  for  their  expression  in 
the  developing  soma. 

9.  GYNANDROMORPHS  AND  SEX  INTERGRADES 

In  insects  and  Crustacea  abnormal  individuals  occa- 
sionally appear,  presenting  both  male  and  female 


FIG.  91. — "A  gynandromorph  mutillid  wasp,  Pseudomethoca  cana- 
densis,  male  on  right  side,  female  on  left."  From  Morgan's 
"Heredity  and  Sex,"  by  permission  of  the  Columbia  University 
Press. 


THE  DETERMINATION  OF  SEX 

characters.  Sometimes  the  demarcation  is  exactly 
median,  one-half  being  male  and  the  other  female. 
Such  forms  are  true  gynandromorphs.  (Fig.  91.) 
There  are  cases,  however,  where  the  division  may  be 
either  dorso-ventral  or  antero-posterior,  and  still 
others  which  show  a  patchwork  of  male  and  female 
parts,  these  latter  being  mosaic  or  inter-sex  individuals. 
Examples  of  such  sex-intergrades  have  been  found 
among  moths  as  described  by  Goldschmidt  and  by 
Banta  among  daphnids. 

Insect  gynandromorphs  do  not  necessarily  have  the 
gonad  of  the  corresponding  sex  in  their  respective 
halves,  showing  that  the  soma  is  not  moulded  by  sex- 
hormones. 

The  cause  of  gynandromorphism  has  been  studied  by 
Boveri  and  by  Morgan.  Boveri  claims  to  have  found 
in  gynandromorph  bees  of  crossed  races  that  the  male 
half  was  maternal,  and  the  female  half  hybrid.  Obvi- 
ously, if  after  the  division  of  the  egg-nucleus,  a  sperm 
unites  with  one  of  the  daughter  nuclei  that  half  will  be 
female,  whereas  the  sister  nucleus,  developing  partheno- 
genetically,  will  form  a  male  half  purely  maternal  in 
origin. 

This  explanation  certainly  holds  good  for  some  cases 
but  Morgan  finds  in  Drosophila  that  male  portions  of 
gynandromorphs  often  bear  paternal  characters,  genes 
of  which  are  in  chromosomes  other  than  the  #-chromo- 
some.  He  concludes,  therefore,  that  at  times  an  ^-chro- 
mosome is  lost  during  the  meiosis  of  a  female  zygote, 
leaving  a  nucleus  that  fails  to  get  two  ^-chromosomes, 
which,  consequently,  develops  into  the  male  portion  of 
the  gynandromorph. 


292  GENETICS 

Similarly  a  misplaced  ^-chromosome  in  a  primary 
germ-cell  may  cause  testes  to  form  in  a  female.  Such 
a  case  of  gynandromorphism  in  Thelia  (Kornhauser) 
proved  upon  actual  chromosome  count  to  have  one 
^-chromosome  missing. 

It  is  rather  difficult  to  offer  any  simple  mechanical 
explanation  for  the  mosaics  or  sex-intergrades  of 
moths  and  daphnids.  Goldschmidt  has  attempted  to 
explain  his  results  upon  a  quantitative  basis,  assign- 
ing values  for  the  determiners  for  maleness  and  female- 
ness,  and  adding  the  assumption  that  the  strength  of 
these  determiners  varies  in  different  races.  Thus,  the 
crossing  of  a  strong  male  race  with  a  weak  male  race 
brings  about  an  upset  of  normal  conditions,  establish- 
ing a  new  balance  of  factors  so  that  neither  one  sex  nor 
the  other  predominates.  An  expression  of  two  sets  of 
genes,  therefore,  is  brought  about  in  various  parts  of 
the  organism. 

Bridges*  recent  work  on  triploid  races  of  Drosophila 
seems  to  indicate  that  when  the  normal  relation  of  the 
autosomal  genes  to  the  sex-genes  of  the  ^-chromosome 
is  upset,  either  by  the  preponderance  of  one  or  the 
other,  then  sex  abnormalities  of  many  sorts  may  be 
expected. 

10.  HEEMAPHEODITISM 

One  of  the  most  obscure  problems  of  the  entire  sex 
question  is  that  of  hermaphroditism,  or  the  production 
of  ova  and  sperm  by  a  single  individual.  Instances  of 
this  condition  are  found  normally  occurring  in  many 
groups  of  invertebrates,  such  as  coelenterates,  cteno- 


THE  DETERMINATION  OF  SEX        293 

phores,  flat-worms,  round-worms,  annelids,  molluscs 
and  crustaceans.  It  is,  however,  the  exception  rather 
than  the  rule  and  must  be  viewed  as  a  modification  of 
the  bisexual  condition  necessitated  to  insure  insemina- 
tion in  animals  poorly  adapted  to  bring  about  typical 
fertilization  of  the  eggs. 

Sometimes  hermaphrodites  are  female  in  appearance 
and  again  they  resemble  more  closely  the  males  of  the 
group  to  which  they  belong.  In  certain  Nematodes, 
for  example,  Rhabdites  aberrant  an  occasional  male  is 
found  among  thousands  of  hermaphrodites  of  female 
appearance.  In  this  worm  Miss  Krueger  has  shown 
that  occasionally  there  is  a  failure  of  one  chromosome 
to  become  incorporated  in  one  of  the  second  sper- 
matocytes.  Spermatozoa  resulting  from  such  deficient 
spermatocytes  may  be  the  cause  of  these  occasional 
male  zygotes.  Since  our  knowledge  of  the  chromosomes 
in  hermaphroditism  is  deficient,  it  is  hardly  worth 
while  at  present  to  speculate  on  the  mechanism  which 
produces  such  individuals. 

That  the  sexual  tendencies  of  hermaphroditic  forms 
are  often  in  a  sensitive  balance,  influenced  by  external 
conditions,  is  shown  by  the  experiments  of  Baltzer  on 
Bonellia  and  by  Gould  on  Crepidula. 

In  the  marine  worm  Bonellia  there  are  produced 
minute  motile  larvae  with  hermaphroditic  possibilities. 
If  these  free-swimming  larvae  find  the  proboscis  of  a 
female  Bonellia  they  attach  themselves  thereto  and 
develop  into  minute  males  after  a  parasitic  existence  of 
about  four  days.  If,  however,  no  proboscis  is  encoun- 
tered, the  motile  larva  sinks  to  the  bottom  and  develops 


294.  GENETICS 

into  a  female.  In  this  case  we  may  say  that  some 
secretion  from  the  female  stimulates  the  development 
of  the  male  potentialities  and  suppresses  those  of  the 
female.  In  fact,  intermediates  were  produced  by 
Baltzer  by  allowing  larvae  to  become  attached  to  a 
proboscis  temporarily  and  then  removing  them  at  in- 
tervals of  less  than  four  days. 

In  Crepidula  plana,  a  hermaphroditic  gasteropod 
which  is  normally  protandric,  that  is,  producing  first 
sperm  and  afterward  ova,  Gould  has  shown  that  the 
presence  of  older  individuals  during  the  female  phase 
of  development  causes  the  production  of  sperm  in  such 
young  individuals  as,  when  isolated  omit  sperm  produc- 
tion, developing  instead  the  female  phase  and  pro- 
ducing ova.  Here  there  is  an  animal  in  sensitive  bal- 
ance influenced  by  a  secretion  which  probably  comes  to 
it  through  the  sea-water  from  individuals  in  the  female 
phase  of  reproduction. 

The  problem  of  hermaphroditism,  its  mechanism  and 
relationship  to  bisexual  reproduction,  is  well  worthy 
of  intensive  study.  From  such  exceptions  to  the  gen- 
eral rule  we  may  hope  to  learn  much  about  the  normal 
mechanism  of  sex-determination. 


11.  CONCLUSION 

Finally,  one  may  ask,  can  sex  ever  be  controlled? 
There  seem  to  be  two  avenues  of  approach  to  this 
problem. 

The  maturation,  and  thereby  sex,  in  forms  in  which 
the  female  is  heterogametic,  may  be  controlled  by  ex- 


THE  DETERMINATION  OF  SEX        295 

ternal  conditions,  as  in  the  case  of  Seller's  moths  and 
Riddle's  doves.  When,  however,  the  male  is  hetero- 
gametic,  it  would  be  possible  to  control  sex  only  by 
some  agency  which  would  differentially  aid  or  inhibit 
the  progress  of  one  of  the  two  kinds  of  sperm  peculiar 
to  this  kind  of  an  organism  in  its  approach  to,  or 
penetration  of,  the  ovum. 


CHAPTER  XIV 

THE  APPLICATION  TO  MAN 
1.  THE  APPLICATION  OF  GENETICS  TO  MAN 

HUMAN  civilization  goes  hand  in  hand  with  the  de- 
gree of  successful  interference  which  man  exerts  upon 
the  natural  forces  surrounding  him. 

Primitive  man  was  overwhelmed  and  outmastered 
by  his  environment,  but  civilized  man  harnesses  nature 
to  do  his  will.  Savages  are  not  proficient  in  the  arts 
of  cultivating  plants  and  domesticating  animals,  while 
these  are  the  very  things  upon  which  human  progress 
fundamentally  depends.  The  degree  of  civilization  of 
any  people  is  closely  correlated  with  the  degree  of  their 
success  in  exercising  a  conquering  control  over  plants 
and  animals.  Any  knowledge  of  the  laws  of  heredity, 
therefore,  as  applied  by  man,  either  directly  to  himself 
or  indirectly  to  animals  and  plants,  is  a  distinct  con- 
tribution to  human  progress. 

In  1900  the  National  Association  of  British  and 
Irish  Millers,  as  Kellicott  points  out,  being  dissatisfied 
with  the  quality  and  quantity  of  the  annual  wheat 
yield,  engaged  Professor  Biffen  to  apply  his  knowledge 
of  heredity  to  the  practical  problem  of  improving  their 
wheat  crop.  The  characters  desired  were  a  short  full 
head,  beardlessness,  high  gluten  content,  immunity  to 

296 


THE  APPLICATION  TO  MAN  297 

rust,  strong  supporting  straw,  and  a  large  yield  per 
acre.  In  the  short  time  that  has  elapsed,  Professor 
Biffen  has  succeeded  in  producing  strains  of  wheat  that 
combine  all  these  desirable  characters  to  a  remarkable 
degree.  Such  an  immediate  result  would  not  have  been 
possible  before  1900,  when  the  rediscovery  of  Mendel's 
law  revolutionized  man's  knowledge  of  the  action  of 
heredity  in  nature. 

This  same  knowledge  which  has  made  possible  the 
improvement  of  wheat  may  be  applied  with  certain 
reservations  to  the  breeding  of  man,  for  there  is  no 
reasonable  doubt  that  man  belongs  in  the  same  evolu- 
tionary series  with  all  other  animals,  as  Darwin  showed, 
and  is  consequently  subject  to  the  same  natural  laws 
to  a  considerable  degree. 

It  must  be  admitted  that  thus  far  in  the  progress  of 
civilization  more  attention  has  been  directed  to  the 
scientific  breeding  of  animals  and  plants,  little  as  that 
has  been,  than  to  the  scientific  breeding  of  man.  Let 
us  hope  that  the  future  will  have  a  different  story  to 
tell! 

2.  MODIFYING  FACTORS  IN  THE  CASE  OF  MAN 

There  are  certain  qualifying  factors  that  make  the 
problems  of  genetics  somewhat  different  in  the  case  of 
man  than  in  other  organisms. 

For  example,  mankind  has  come  to  be  partially  ex- 
empt from  some  of  the  natural  laws  which  affect  other 
organisms.  Thus  with  respect  to  the  workings  of 
natural  selection  man  is  partially  under  "grace"  rather 
than  "law."  Nature  no  longer  "selects"  good  eyes  in 


298  GENETICS 

man  by  long,  patient,  and  devious  processes  when  poor 
eyes  are  made  good  almost  instantly  by  a  visit  to  the 
oculist.  She  has  long  since  given  up  providing  natu- 
ral weapons  of  defense  for  those  who  have  the  wits  to 
supply  themselves  more  efficiently  with  artificial  means 
of  self-preservation,  and  she  no  longer  attempts  to 
improve  the  natural  powers  of  locomotion  of  those 
who  are  able  to  tame  a  horse  to  ride  upon,  or  who 
build  steamships,  railroads,  automobiles  and  aero- 
planes, thus  accomplishing  at  once  what  would  require 
ages  at  least  to  evolve. 

Neither  does  the  law  of  the  survival  of  the  fittest  in 
its  original  sense  apply  equally  to  man  and  to  other 
organisms.  Human  society  to-day  protects  its  unfit 
in  hospitals,  asylums,  and  through  various  philan- 
thropies, while  physicians  devote  themselves  to  the  art 
of  prolonging  life  beyond  the  period  of  usefulness. 

We  do  not  desire  these  results  of  our  modern 
civilization  to  be  otherwise,  but  the  fact  remains  that 
some  of  the  most  inflexible  and  universal  "natural 
laws"  are  ineffective  in  the  case  of  man,  and  it  is  profit- 
able to  bear  this  in  mind  when  applying  the  laws  of 
genetics  to  man. 

The  laboratory  for  human  heredity  is  the  wide 
world,  and  it  is  obvious  that  the  experimental  method 
which  has  proven  so  effective  in  studying  the  heredity 
of  animals  and  plants  is  impracticable  in  the  case  of 
man.  The  consideration  of  human  heredity,  therefore, 
must  always  be  largely  from  the  statistical  side,  con- 
sisting in  an  analysis  of  experiments  already  performed 
rather  than  in  arbitrarily  initiating  new  experiments. 

Such  institutions  as  insane  asylums,  prisons,  sani- 


THE  APPLICATION  TO  MAN  299 

tariums,  and  homes  for  the  unfortunate  are  excellent 
foci  for  studying  certain  phases  of  human  heredity, 
because  they  are  simply  convenient  places  where  the 
results  of  similar  dysgenic  experiences  have  been 
brought  together. 

3.  EXPERIMENTS  IN  HUMAN  HEREDITY 
A.  THE  JUKES 

A  classic  example  of  an  experiment  in  human  hered- 
ity which  has  been  partially  analyzed  by  the  statistical 
method  is  that  furnished  by  Dugdale  in  1877  in  the 
case  of  "Max  Jukes"  and  his  descendants.  At  that 
time  it  included  over  one  thousand  individuals,  the 
origin  of  all  of  whom  has  been  traced  back  to  a  shift- 
less, illiterate,  and  intemperate  backwoodsman  who 
started  his  experiment  in  heredity  in  western  New  York 
when  it  was  yet  an  unsettled  wilderness. 

In  1877  the  histories  of  540  of  this  man's  progeny 
were  known,  and  that  of  most  of  the  others  was  partly 
known.  About  one  third  of  this  degenerate  strain  died 
in  infancy,  310  individuals  were  paupers  who  all  to- 
gether spent  a  total  of  2300  years  in  almshouses,  while 
440  were  physical  wrecks.  In  addition  to  this,  over 
one  half  of  the  female  descendants  were  prostitutes, 
and  130  individuals  were  convicted  criminals,  including 
7  murderers.  Not  one  of  the  entire  family  had  a  com- 
mon school  education,  although  the  children  of  other 
families  in  the  same  region  found  a  way  to  educational 
advantages.  Only  20  individuals  learned  a  trade  and 
10  of  these  did  so  in  state's  prison. 

It  is  estimated  that  up  to  1877  this  experiment  in 


300  GENETICS 

human  breeding  had  cost  the  state  of  New  York  over 
a  million  and  a  quarter  dollars,  not  including  the  drink 
bill,  and  the  end  is  by  no  means  yet  in  sight. 

The  discovery  in  1911  of  Dugdale's  original  manu- 
script giving  the  real  names  and  localities  of  the  mem- 
bers of  the  Jukes  clan  made  it  possible  to  follow  up 
the  later  history  of  this  famous  strain  of  undesirable 
human  germplasm.  This  was  done  by  Dr.  A.  H. 
Esterbrook,  who  published  the  results  of  his  investiga- 
tions under  the  title  of  "The  Jukes  in  1915,"  *  after 
personally  visiting  every  individual  whom  he  was  able 
to  trace. 

Since  Dugdale's  time  the  Jukes,  now  in  the  eighth 
generation,  have  been  forced  to  disperse  from  their 
original  habitat  because  the  cement  mining  industry 
upon  which  most  of  them  formerly  depended  for  a 
livelihood  was  abandoned  with  the  introduction  of 
Portland  cement.  Esterbrook  has  recorded  2094  indi- 
viduals bearing  Jukes'  blood  who  were  scattered 
through  fourteen  states.  Of  748  living  descendants  of 
Max  Jukes  over  15  years  of  age,  he  found  76  who  were 
socially  adequate;  255  doing  fairly  well;  323  "typical 
degenerates,"  and  94  whom  he  left  unclassified  due  to 
lack  of  sufficient  information.  He  says : — "The  re- 
moval of  Jukes  from  their  original  habitat  to  new  re- 
gions is  beneficial  to  the  stock  itself,  as  better  social 
pressure  is  brought  to  bear  on  them  and  there  is  a 
chance  for  mating  into  better  families,"  and  Daven- 
port, commenting  on  the  entire  matter,  adds, — "The 
most  important  conclusion  that  may  be  drawn  from 
Carnegie  Institution  of  Washington,  Pub.  240,  1916,  pp.  85. 


THE  APPLICATION  TO  MAN  301 

Dr.  Esterbrook's  prolonged  study  of  the  Jukes  forty 
years  later  is  that  not  merely  institutional  care  nor 
better  environment  will  cause  good  social  reactions  in 
persons  who  are  feeble-minded  or  feebly-inhibited,  al- 
though on  the  other  hand,  better  stimuli  will  secure 
better  reactions  from  weak  stock  than  will  poor  stimuli. 
.  .  .  The  chief  value  of  a  detailed  study  of  this  sort 
lies  in  this :  that  it  demonstrates  again  the  importance 
of  the  factor  of  heredity." 


B.    THE    DESCENDANTS    OF    JONATHAN    EDWAEDS 

In  striking  contrast  to  the  case  of  Max  Jukes  is 
that  of  Jonathan  Edwards,  the  eminent  divine,  whose 
famous  progeny  Winship  describes  as  follows:  "1394 
of  his  descendants  were  identified  in  1900,  of  whom 
295  were  college  graduates;  13  presidents  of  our 
greatest  colleges,  besides  many  principals  of  other  im- 
portant educational  institutions;  60  physicians,  many 
of  whom  were  eminent;  100  and  more  clergymen, 
missionaries,  or  theological  professors ;  75  were  officers 
in  the  army  and  navy ;  60  were  prominent  authors  and 
writers,  by  whom  135  books  of  merit  were  written  and 
published  and  18  important  periodicals  edited;  33 
American  states  and  several  foreign  countries  and  92 
American  cities  and  many  foreign  cities  have  profited 
by  the  beneficent  influence  of  their  eminent  activity; 
100  and  more  were  lawyers,  of  whom  one  was  our  most 
eminent  professor  of  law;  30  were  judges;  80  held 
public  office,  of  whom  one  was  vice-president  of  the 
United  States ;  3  were  United  States  senators ;  several 


302  GENETICS 

were  governors,  Members  of  Congress,  framers  of  state 
constitutions,  mayors  of  cities,  and  ministers  to  foreign 
courts ;  one  was  president  of  the  Pacific  Mail  Steamship 
Company;  15  railroads,  many  banks,  insurance  com- 
panies, and  large  industrial  enterprises  have  been  in- 
debted to  their  management.  Almost  if  not  every  de- 
partment of  social  progress  and  of  public  weal  has 
felt  the  impulse  of  this  healthy,  long-lived  family.  It 
is  not  known  that  any  one  of  them  was  ever  convicted 
of  crime." 

Similarly  Galton,  in  "Hereditary  Genius,"  points  out 
in  his  analysis  of  one  hundred  celebrated  persons  that 
they  had  3  great-grandfathers;  17  grandfathers;  31 
fathers;  48  sons;  14  grandsons  and  3  cousins  who  also 
were  celebrated. 

C.    THE    KAI/LIKAK    FAMILY 

A  more  convincing  experiment  in  human  heredity 
than  the  foregoing,  since  it  concerns  the  descendants 
of  two  mothers  and  the  same  father,  is  furnished  by  the 
recently  published  history  of  the  "Kallikak"  family.1 

During  Revolutionary  days,  the  first  Martin  Kalli- 
kak,— the  name  is  fictitious, — who  was  descended  from 
a  long  line  of  good  English  ancestry,  took  advantage 
of  a  feeble-minded  girl.  The  result  of  their  indulgence 
was  a  feeble-minded  son  who  became  the  progenitor  of 
480  known  descendants  of  whom  143  were  distinctly 
feeble-minded,  while  most  of  the  others  fell  below 
mediocrity  without  a  single  instance  of  exceptional 
ability. 

'"The  Kallikak  Family."    H.  H.  Goddard.    The  Macmillan  Co. 


THE  APPLICATION  TO  MAN  303 

"After  the  Revolutionary  war,  Martin  married  a 
Quaker  girl  of  good  ancestry  and  settled  down  to  live 
a  respectable  life  after  the  traditions  of  his  fore- 
fathers. From  this  legal  union  with  a  normal  woman 
there  have  been  496  descendants.  All  of  these  except 
two  have  been  of  normal  mentality  and  these  two  were 
not  feeble-minded.  .  .  .  The  fact  that  the  descendants 
of  both  the  normal  and  the  feeble-minded  mother  have 
been  traced  and  studied  in  every  conceivable  environ- 
ment, and  that  the  respective  strains  have  always  been 
true  to  type,  tends  to  confirm  the  belief  that  heredity 
has  been  the  determining  factor  in  the  formation  of 
their  respective  characters." 

Other  recent  extensive  studies  of  dysgenic  lines  in- 
clude the  "Nams,"  the  "Hill  Folk,"  the  "Pineys"  of 
New  Jersey,  the  "Ishmaels"  of  Indiana  and  the 
"Zeros"  of  Denmark. 


4.  MORAL  AND  MENTAL   CHARACTERS   BEHAVE 
LIKE  PHYSICAL  ONES 

These  instances  of  human  breeding  show  unmistak- 
ably that  "blood  counts"  in  human  inheritance,  even 
though  the  hereditary  unit  characters  that  lead  to 
these  general  results  have  not  yet  been  analyzed  with 
the  clearness  that  is  possible  in  dealing  with  the  char- 
acters of  some  animals  and  plants. 

There  is  of  course  no  question  of  moral  and  mental 
traits  in  plants,  and  the  role  that  these  play  in  animals 
is  not  easy  to  determine;  but  in  man  the  case  is  un- 
doubtedly much  more  important  and  complex,  since 


304  GENETICS 

mental  and  moral  characteristics  have  a  large  share 
in  making  man  what  he  is.  The  brute  acts  according 
to  his  inherited  organization ;  man  is  urged  by  his  but 
may  act  according  to  a  higher,  moral  law.  There  is, 
however,  no  fundamental  scientific  distinction  which 
can  be  drawn  between  moral,  mental,  and  physical 
traits,  and  they  are  undoubtedly  all  equally  subject  to 
the  laws  of  heredity. 

For  instance,  as  an  illustration  of  the  heritability 
of  non-physical  traits,  in  the  Jukes  pedigree  three  of 
the  daughters  of  Max  impressed  their  peculiar  moral 
and  mental  characteristics  in  a  distinctive  way  upon 
their  offspring.  To  quote  Davenport:  "Thus  in  the 
same  environment,  the  descendants  of  the  illegitimate 
son  of  Ada  are  prevailingly  criminal;  the  progeny  of 
Belle  are  sexually  immoral;  and  the  offspring  of  Effie 
are  paupers.  The  difference  in  the  germplasm  deter- 
mines the  difference  in  the  prevailing  trait."  As 
Woods  observes :  "The  most  interesting  and  even 
startling  thing  has  been  the  ease  with  which  heredity 
has  been  able  to  bear  the  brunt  of  explaining  the  gen- 
eral make-up  of  character." 

5.  THE  CHARACTER  OF  HUMAN  TRAITS 

Of  the  mental,  moral,  and  physical  traits  which  are 
heritable  in  man,  some  must  be  regarded  as  generally 
desirable,  some  as  indifferent,  and  others  as  defects  to 
be  avoided  if  possible.  In  general  the  majority  of 
human  traits,  those  which  together  make  up  man  as 
distinguished  from  other  animals,  do  not  particularly 


THE  APPLICATION  TO  MAN  305 

claim  the  attention  because  they  are  so  universal. 
Some  which  stand  out  from  the  mass,  such  as  the 
physical  traits  of  eye-color  and  the  color  and  charac- 
ter of  hair,  may  be  regarded  as  indifferent  so  far  as 
the  welfare  of  the  individual  is  concerned,  while  others 
like  skin  color  and  certain  racial  features  that  charac- 
terize particular  strains  of  "blood"  may,  under  certain 
circumstances,  work  a  social  handicap  upon  their  pos- 
sessors according  to  the  traditions  of  the  community  in 
which  they  appear. 

A  long  list  of  desirable  mental  traits  might  be  enu- 
merated that  seem  in  a  general  way  to  be  subject  to 
the  laws  of  inheritance,  although  they  have  not  yet 
undergone  the  careful  analysis  demanded  by  modern 
genetics  which  deals  in  unit  characters  rather  than  in 
lump  inheritance. 

Musical,  literary,  or  artistic  ability,  for  example, 
mathematical  aptitude  and  inventive  genius,  as  well  as 
a  cheerful  disposition  or  a  strong  moral  sense  are 
probably  all  gifts  that  come  in  the  germplasm.  They 
may  each  be  developed  by  exercise  or  repressed  by 
want  of  opportunity,  nevertheless  they  are  fundamen- 
tally germinal  gifts. 

A  genius  must  be  born  of  potential  germplasm. 
There  are  no  "self-made  men."  Each  has  within  from 
his  ancestry,  the  potentiality  of  whatever  he  becomes. 
No  amount  of  faithful  plodding  application  can  com- 
pensate for  a  lack  of  the  divine  hereditary  spark  at 
the  start. 


306  GENETICS 

6.  HEREDITARY  DEFECTS 

Undesirable  hereditary  traits  are  frequently  defects 
due  to  the  absence  of  some  character.  For  instance, 
albinism,  which  occurs  in  several  kinds  of  animals  and 
also  in  man  in  one  out  of  every  20,000  individuals 
(according  to  Elderton),  is  due  to  the  absence  of  pig- 
ment in  the  skin,  hair  and  eyes.  Albinic  individuals 
have  poor  eyesight  because  they  are  unable  to  stand 
strong  light,  being  without  protective  pigment  in  the 
eyes.  This  peculiarity  of  albinism  behaves  as  a  reces- 
sive character  both  in  man  and  in  other  animals.  An 
albinic  individual  may,  therefore,  marry  a  normal  indi- 
vidual without  fear  of  producing  albino  children,  al- 
though the  children  of  such  a  mating  would  carry 
heterozygous  germplasm  with  respect  to  albinism,  and 
in  cousin  marriages  might  subsequently  produce  some 
albino  children. 

Davenport,  in  his  work  on  "Heredity  in  Relation  to 
Eugenics,"  brings  together  a  long  catalogue  of  human 
hereditary  defects,  although  in  most  instances  they 
are  extremely  difficult  of  accurate  analysis.  This  is 
true,  first,  because  these  defects  so  often  probably  de- 
pend upon  a  combination  of  determiners  rather  than 
upon  a  single  one,  and,  second,  because  the  available 
data  are  usually  scattered  and  incomplete. 

Deafness,  for  example,  is  a  defect  which  is  heredi- 
tary though  exactly  to  what  degree,  it  is  at  present 
impossible  to  state.  The  following  table  taken  from 
the  extensive  work  of  Fay  (1898)  upon  "Marriage  of 
the  Deaf  in  America"  gives  some  idea  of  the  results  of 
different  matings  lumped  together  statistically. 


THE  APPLICATION  TO  MAN  307 


CONDITION  OF  PARENTS 

PERCENTAGE  OF 
DEAF  OFFSPRING 

Both   born   deaf       .           .           .           ... 

25.9 

One  born  deaf,  one  with  acquired  deafness   . 

6.5 

One  born  deaf    one  normal                           . 

11.9 

Both  with  acquired  deafness  

2.3 

One  with  acquired  deafness,  one  normal     .     . 

2.2 

That  two  parents  born  deaf  do  not  produce  more 
than  26  per  cent  of  deaf  children  is  probably  due  to 
the  fact,  first,  that  each  parent  is  in  all  likelihood  het- 
erozygous for  deafness  and  that,  second,  the  same  com- 
bination of  factors  which  is  the  cause  of  the  parental 
defect  on  either  side  of  the  pedigree  does  not  happen  to 
recombine  after  segregation  to  form  the  new  individual. 
Deafness  will  be  produced  in  the  offspring  only  when 
matings  occur  in  which  the  proper  factors  are  com- 
bined. Such  an  undesirable  result  is  much  more  likely 
to  happen  if  both  parents  come  from  the  same,  or 
related,  hereditary  strains  than  if  they  are  derived 
from  families  in  no  way  connected  by  blood. 

Herein  lies  the  biological  objection  to  cousin  mar- 
riage which  tends  to  bring  together,  and  thus  to  per- 
petuate, like  defects.  Outcrossing,  on  the  contrary, 
through  the  law  of  dominance,  tends  to  conceal  defects 
and  to  prevent  their  expression. 

If  the  patent  parental  characters  were  all  that  re- 
appeared in  the  offspring,  the  marriage  of  near  kin 
would  present  fewer  difficulties.  It  is  the  "skeleton  in 
the  closet"  that  makes  trouble.  Elderton  gives  a  case 
of  haemophilia  where  the  direct  line  was  free  from  taint 


308  GENETICS 

but  collaterals  showed  the  disease  latent  for  six  gen- 
erations. 

Inbreeding  is  often  the  result  of  proximity.  Insular 
or  isolated  communities,  slums  in  cities,  where  those  of 
one  language  herd  together,  or  hovels  in  the  back- 
woods, where  degenerates  of  a  kind  are  kept  in  intimate 
association,  as  well  as  asylums  of  various  sorts  in 
which  similar  defectives  are  promiscuously  housed 
under  the  same  roof,  are  all  potent  agencies  to  insure 
human  inbreeding. 

Similarly,  localities  which  have  been  devastated  by 
migrations  of  the  most  effective  blood,  as,  for  example, 
parts  of  Ireland  or  many  rural  villages  in  New  Eng- 
land, are  frequently  characterized  by  a  population 
showing  a  large  percentage  of  defectiveness.  The  able- 
bodied  and  ambitious  go  forth  into  the  world  to  seek 
their  fortunes,  while  the  deficient  in  body  or  spirit  are 
left  behind  where,  under  the  spell  of  proximity,  they 
perpetuate  their  deficiencies. 

The  part  that  improved  transportation  has  played 
in  mixing  up  populations  and  in  counteracting  the 
effects  of  stagnation  on  human  heredity,  through  in- 
breeding under  the  inertia  of  proximity,  is  very  great. 
There  were,  obviously,  geographic  reasons  for  the 
well-known  love  story  of  Adam  and  Eve.  Before  the 
days  of  railroads,  cousin-marriages  were  much  more 
frequent  than  they  are  now. 

Many  cases  of  human  defects,  such  as  imbecility  or 
insanity,  are  extremely  difficult  of  analysis  from  the 
standpoint  of  heredity  because,  in  the  first  place,  the 
defective  conditions  descriptively  included  under  these 


THE  APPLICATION  TO  MAN  309 

vague  terms  are  made  up  of  a  multitude  of  diverse  con- 
ditions each  of  which  must  have  a  different  array  of 
determiners  and,  in  the  second  place,  because  any  one 
definite  sort  of  insanity  or  imbecility  may  be  condi- 
tioned by  a  variety  of  factors. 

However,  the  difficulty  of  the  problem  is  no  reason 
for  abandoning  the  attempt  to  reach  its  solution  and 
to  learn,  if  possible,  "whence  come  our  300,000  insane 
and  feeble-minded,  our  160,000  blind  or  deaf,  the 
2,000,000  that  are  annually  cared  for  by  our  hospitals 
and  homes,  our  80,000  prisoners  and  the  thousands  of 
criminals  that  are  not  in  prison,  and  our  100,000  pau- 
pers in  almshouses  and- out"  (Davenport). 

7.  THE  CONTROL  OF  DEFECTS 

The  method  of  possible  control  of  human  defects 
depends  upon  whether  they  are  positive  or  negative, 
that  is,  dominant  or  recessive.  In  those  cases  where 
a  given  defect  is  due  to  a  single  determiner  the 
Mendelian  expectation  for  the  possible  offspring 
arising  from  various  matings  is  indicated  in  the 
table  on  page  310  in  which  D  stands  for  the  defect 
and  d  for  its  absence. 

If  the  defect  is  positive  and  in  a  duplex  or  homo- 
zygous  condition  in  one  parent,  as  in  1,  2,  and  4 
all  the  offspring  will  possess  it  regardless  of  the  ger- 
minal constitution  of  the  other  parent.  In  two  cases 
only,  namely,  in  3  and  5,  where  the  defective  parent  is 
heterozygous,  is  there  any  chance  of  unaffected  off- 
spring, and  even  in  these  cases  the  defect  is  quite  as 


310  GENETICS 

THE  MEXDELIAX  EXPECTATION-  FOR  DEFECTS 


IF  THE  DEFECT  is  POSITIVE 
(dominant) 

IF  THE  DEFECT  is  NEGATIVE 
(recessive) 

When  both 
parents  show 
the  defect 

1 

DDXDD=allDD 

dd  X  dd=all  dd 

2 

DDXDd=$DD  +  lDd 

3 

Dd  X  Dd=\DD  +  %Dd  +  \dd 

When  one 
parent  only 
shows  the 
defect 

When  neither 
parent  shows 
the  defect 

4 

DD  X  dd=a\l  Dd 

dd  X  DD=all  Dd 

5 

DdXdd=$Dd  +  ldd 

ddXDd=$Dd  +  $dd 

6 

ddXdd=alldd 

DDXDD=sdlDD 

7 

DdXDD  =  $DD  +  $Dd 

8 

DdXDd=±DD  +  lDd  +  \dd 

likely  to  appear  as  not.  It  is  obvious  that  the  only 
way  to  rid  germplasm  of  a  dominant  defect  is  by  con- 
tinued mating  with  recessive  individuals.  By  this 
method  it  is  possible  in  time  to  shake  off  the  defect. 
When  it  once  disappears  in  any  individual,  it  will  never 
return  unless  crossed  back  to  a  similar  defective  domi- 
nant strain. 

In  other  words,  such  a  recessive  extracted  from  a 
heterozygous  ancestry  will  breed  just  as  true  as  a 
recessive  which  was  pure  from  the  start.  In  both  in- 
stances there  is  an  entire  absence  of  the  character  in 
question,  and  it  is  clear  that  this  character  can  there- 
after never  again  reappear,  since  something  cannot  be 
derived  from  nothing. 

On  the  other  hand,  if  a  defect  is  negative,  depending 
upon  the  absence  of  a  normal  dominant  determiner,  as 
is  usually  the  case  with  defects,  it  behaves  as  a 
Mendelian  recessive,  that  is,  it  is  always  apparent  in 
individuals  developing  from  the  homozygously  defective 
germplasm. 


THE  APPLICATION  TO  MAN 


311 


It  is  certain,  for  example,  that  an  imbecile  which  has 
arisen  from  homozygous  defective  germplasm  carries 
only  the  determiner  for  imbecility  in  his  own  germ- 
plasm,  and  when  two  such  recessives  mate,  nothing  but 
imbecile  offspring  can  result,  for  recessives  breed  true. 
Nothing  plus  nothing  equals  nothing. 

For  practical  purposes  it  is  unimportant  to  know 


FIG.  92. — Pedigree  chart  illustrating  the  law  that  two  defective 
parents  have  only  defective  offspring.  A,  alcoholic;  (7, 
criminalistic ;  d,  died;  Ff  feeble-minded;  T,  tubercular. 
After  Goddard. 

whether  or  not  feeble-mindedness,  or  any  similar  defect, 
is  Mendelian  in  behavior.  The  fact  that  it  is  heredi- 
tary is  enough. 

An  illustration  of  this  principle  is  given  in  the 
above  pedigree  (Fig.  92)  furnished  by  Goddard,  1910. 
The  result  is  quite  different,  however,  when  one  parent 
only  shows  the  defect.  If  the  other  parent  is  a  normal 
homozygote,  as  in  Case  4  of  the  accompanying  table, 
all  the  offspring  will  be  normal  in  appearance,  but  with 
the  bar  sinister  of  defectiveness  in  their  germplasm, 
while  if  it  is  heterozygous  (Case  5),  one  half  of  the 
progeny  will  be  defective. 


312  GENETICS 

Finally,  when  neither  parent  shows  defectiveness  but 
one  carries  the  defect  as  a  heterozygote  (Case  7),  then 
there  will  be  no  defective  children,  while  if  both  parents 
are  heterozygous  there  is  one  chance  in  four  that  the 
offspring  will  be  defective. 

As  a  matter  of  fact,  defectives  usually  mate  with 
defectives  for  the  simple  reason  that  normals  ordi- 
narily avoid  them,  so  it  comes  about  that  streams  of 
poor  germplasm  naturally  flowing  together  tend  to  the 
inbreeding  of  like  defects. 

Davenport  *  lays  down  the  following  general  eugenic 
rules  for  the  guidance  of  those  who  would  produce 
offspring  wisely:  "If  the  negative  character  is,  as  in 
polydactylism  and  night-blindness,  the  normal  char- 
acter, then  normals  should  marry  normals,  and  they 
may  be  even  cousins.  If  the  negative  character  is 
abnormal,  as  imbecility  and  liability  to  respiratory 
diseases,  then  the  marriage  of  two  abnormals  means 
probably  all  children  abnormal;  the  marriage  of  two 
normals  from  defective  strains  means  about  one  quar- 
ter of  the  children  abnormal;  but  the  marriage  of  a 
normal  of  the  defective  strain  with  one  of  a  normal 
strain  will  probably  lead  to  strong  children.  The 
worst  possible  marriage  in  this  class  of  cases  is  that 
of  cousins  from  the  defective  strain,  especially  if  one 
or  both  have  the  defect.  In  a  word,  the  consanguineous 
marriage  of  persons  one  or  both  of  whom  have  the  same 
undesirable  defect,  is  highly  unfit,  and  the  marriage 
of  even  unrelated  persons  who  both  belong  to  strains 

1  Davenport.    Rep.  of  Amer.  Breeder*'  A*»oc.,  Vol.  VI,  p.  431, 
1910. 


THE  APPLICATION  TO  MAN  313 

containing  the  same  undesirable  defect  is  unfit.  Weak- 
ness in  any  characteristic  must  be  mated  with  strength 
in  that  characteristic;  and  strength  may  be  mated 
with  weakness." 

In  short,  the  eugenical  Cupid  does  not  tell  one  so 
often  whom  to  select  for  a  partner  as  whom  to  avoid. 


CHAPTER  XV 

HUMAN  CONSERVATION 

1.  How  MANKIND  MAY  BE  IMPROVED 

THERE  are  two  fundamental  ways  to  bring  about 
human  betterment,  namely,  by  improving  the  indi- 
vidual and  by  improving  the  race.  The  first  method 
consists  in  making  the  best  of  whatever  heritage  has 
been  received  by  placing  the  individual  in  the  most 
favorable  environment  and  developing  his  capacities 
to  the  utmost  through  education.  Such  enterprises 
may  be  included  under  this  head  as  improving  sanita- 
tion, controlling  disease,  insuring  health,  safe-guarding 
human  life,  banishing  child-labor,  lessening  drudgery 
of  all  kinds,  substituting  something  better  for  the 
slums,  championing  the  weak,  reforming  penal  institu- 
tions, maintaining  charitable  organizations,  cultivat- 
ing true  temperance,  dispelling  ignorance  and  length- 
ening life.  The  second  method  consists  in  seeking  a 
better  heritage  with  which  to  begin  the  life  of  the 
individual. 

The  first  method  is  immediate  and  urgent  for  the 
present  generation.  The  second  method  is  concerned 
with  ideals  for  the  future,  and  consequently  does  not 
usually  present  so  strong  an  appeal  to  the  individual, 

314 


HUMAN  CONSERVATION  315 

The  first  is  the  method  of  euthenics,  or  the  science 
of  learning  to  live  well.  The  second  is  eugenics,  which 
Galton  defines  as  "the  science  of  being  well  born." 
Every  gain  in  eugenics,  it  need  hardly  be  said,  will 
make  euthenics  more  effective  but  the  reverse  cannot 
be  affirmed. 

These  two  aspects  of  human  betterment,  however, 
are  inseparable.  Any  hereditary  characteristic  must 
be  regarded,  not  as  an  independent  entity,  but  as  a 
reaction  between  the  germplasm  and  its  environment. 
The  biologist  who  disregards  the  fields  of  educational 
endeavor  and  environmental  influence,  is  equally  at 
fault  with  the  sociologist  who  fails  sufficiently  to  real- 
ize the  fundamental  importance  of  the  germplasm. 

Without  euthenic  opportunity  the  best  of  heritages 
would  never  fully  come  to  its  own.  Without  the 
eugenic  foundation  the  best  opportunity  fails  of  ac- 
complishment. The  euthenic  point  of  view,  however, 
must  not  distract  the  attention  now,  for  the  present 
chapter  is  particularly  concerned  with  the  program  of 
eugenics. 

2.  HUMAN  ASSETS  AND  LIABILITIES 

In  an  attempt  to  take  account  of  human  stock 
Dr.  H.  H.  Laughlin,  of  the  Eugenics  Record  Office, 
has  made  the  following  eugenical  classification  based 
on  the  manner  in  which  families  assemble  in  their  off- 
spring heritable  traits  which  determine  for  their  pos- 
sessors (a)  social  adjustment  and  (b)  special  talent 
or  defect. 


316  GENETICS 

I.  Persons  of  genius; 

II.  Persons   of   special   skill,   intelligence,   courage, 
unselfishness,  enterprise  or  strength; 

III.  Persons   constituting  the  great   normal  middle 

class,  the  "people"; 

IV.  Socially  inadequate  persons. 

The  first  three  groups  constitute  those  eugenically 
fit  from  sterling  inheritance,  who  produce  the  socially 
valuable  nine-tenths  of  humanity  among  civilized  peo- 
ple, and  in  the  last  group  are  the  eugenically  unfit  from 
defective  inheritance  who  produce  the  socially  inade- 
quate or  the  "submerged  tenth"  of  humanity. 

Among  persons  of  genius  Dr.  Laughlin  would  in- 
clude the  5000  persons  most  splendidly  equipped  by 
nature  throughout  historic  times,  as,  for  example, 
Aristotle  in  philosophy,  Newton  in  science,  Pasteur  in 
medicine,  Dante  in  poetry,  Shakespeare  in  drama,  and 
Cecil  Rhodes  in  business.  Reckoning  that  since  civili- 
zation began  there  have  been  born  and  reared  in  civil- 
ized countries  approximately  thirty  billion  persons,  the 
expectation  of  a  genius  is  about  1  :  6,000,000. 

In  the  second  group  are  included  the  "natural  and 
acknowledged  leaders  in  all  lines  of  human  endeavor, — 
the  "Who's  Who  people."  The  incidence  of  these  in 
the  total  population  is  possibly  1  :  6,000. 

The  third  group,  the  "people,"  constitute  nine- 
tenths  of  all,  since  the  first  two  classes,  although  their 
influence  is  very  great,  are  numerically  negligible, 
while  the  fourth  group  is  made  up  of  the  residue  or  the 
socially  inadequate,  namely,  (1)  feeble-minded;  (2) 


HUMAN  CONSERVATION  317 

pauper;  (3)  inebriate;  (4)  criminalistic ;  (5)  epileptic; 
(6)  insane;  (7)  asthenic  or  weak;  (8)  diathetic,  or 
predisposed  to  disease;  (9)  deformed;  (10)  cacses- 
thenic,  that  is,  with  defective  sense  organs. 

Laughlin  concludes ; — "The  task  of  eugenics  is  (1)  to 
encourage  fit  and  fertile  matings  among  those  persons 
most  richly  endowed  by  nature  and  (2)  to  devise  prac- 
ticable means  for  cutting  off  the  inheritance  lines  of 
persons  of  naturally  meagre  or  defective  inheritance." 

3.  MORE  FACTS  NEEDED 

Since  the  point  of  attack  in  human  heredity  must 
be  largely  statistical,  it  is  of  the  first  importance  to 
collect  more  facts.  Our  actual  knowledge  is  confused 
with  a  mass  of  tradition  and  opinion,  much  of  which 
rests  upon  questionable  foundations.  The  great  pres- 
ent need  is  to  learn  more  facts ;  to  sift  the  truth  from 
error  in  what  is  already  known;  and  to  reduce  all 
these  data  to  workable  scientific  form.  Much  progress 
is  being  made  in  this  direction,  owing  to  the  impetus 
given  by  the  revival  of  Mendel's  illuminating  work,  but 
as  yet  the  science  of  eugenics  is  in  its  infancy. 

Eugenics,  being  a  biological  science,  its  truths  can- 
not be  arrived  at  by  arbitration  and  discussion,  and  no 
doubt  the  entire  eugenic  movement  has  suffered  much 
at  the  hands  of  its  over-enthusiastic  friends.  There  is 
a  wide  difference  between  eugenic  zeal  and  eugenic 
knowledge  and  wisdom. 

"If  there  is  one  thing  to  be  deprecated  little  less  than 
ignorance  or  indifference,"  says  Sir  John  MacDonald, 


318  GENETICS 

"it  is  science  in  a  hurry, — eagerness  to  go  to  market 
with  one's  crops  before  they  are  fully  ripe." 

The  most  systematic  and  effective  attempt  in  this 
country  to  collect  reliable  data  concerning  heredity 
in  man  has  been  initiated  under  the  leadership  of 
Dr.  C.  B.  Davenport  in  connection  with  what  is  now 
the  Department  of  Genetics  of  the  Carnegie  Institu- 
tion of  Washington.  This  began  in  1910  as  the  Eu- 
genics Record  Office,  with  a  staff  of  expert  field  and 
office  workers  and  an  adequate  equipment  of  fire-proof 
vaults,  etc.,  for  the  preservation  of  records,  at  Cold 
Spring  Harbor,  Long  Island,  New  York,  under  Dr. 
H.  H.  Laughlin  as  superintendent.  "The  main  work  of 
this  office  is  investigation  into  the  laws  of  inheritance 
of  traits  in  human  beings  and  their  application  to  eu- 
genics. It  proffers  its  services  free  of  charge  to  per- 
sons seeking  advice  as  to  the  consequences  of  pro- 
posed marriage  matings.  In  a  word,  it  is  devoted  to 
the  advancement  of  the  science  and  practice  of  eu- 
genics." Already  a  considerable  number  of  publica- 
tions have  been  issued  from  the  Eugenics  Record 
Office. 

The  Volta  Bureau,  founded  about  thirty-five  years 
ago  in  Washington  by  Dr.  Alexander  Graham  Bell,  is 
collecting  data  with  reference  to  deafness  and  has  now 
systematically  arranged  particulars  concerning  the 
history  of  over  20,000  individuals.  In  England,  also, 
the  Galton  Laboratory  for  Eugenics,  founded  in  1905, 
is  systematically  collecting  facts  about  human  pedi- 
grees and  publishing  the  results  in  a  compendious 
"Treasury  of  Human  Inheritance." 


HUMAN  CONSERVATION  319 

Besides  these  special  bureaus  of  investigation,  innu- 
merable facts  about  the  inheritance  of  particular  traits 
are  being  incidentally  brought  together  and  made  avail- 
able in  various  institutions  and  asylums  throughout 
the  world  immediately  concerned  with  the  care  of  de- 
fectives of  different  types.  It  is  in  connection  with 
such  institutions  for  defectives  that  much  of  the  most 
successful  "field  work"  is  being  accomplished  in  the 
United  States. 

4.  FURTHER  APPLICATION  OF  WHAT  WE  KNOW 
NECESSARY 

Human  performance  always  lags  behind  human 
knowledge.  Many  persons  who  are  fully  aware  of  the 
right  procedure  do  not  put  their  knowledge  into  prac- 
tice. It  follows,  therefore,  that  any  program  of  eu- 
genics which  does  not  grip  the  imagination  of  the 
common  people  in  such  a  way  as  to  become  an  effective 
part  of  their  very  lives  is  bound  to  remain  largely  an 
academic  affair  for  Utopians  to  quarrel  and  theorize 
over. 

It  is  not  enough  to  collect  facts  and  work  out  an 
analysis  and  interpretation  of  them,  for,  important 
as  this  preliminary  step  is,  it  must  be  followed  by  a 
convincing  campaign  of  education. 

The  lives  of  the  unborn  do  not  force  themselves 
upon  the  average  man  or  woman  with  the  same  insis- 
tency as  lives  already  begun.  In  the  midst  of  the 
overwhelming  demands  of  the  present,  the  appeal  of 
posterity  for  better  blood  is  vague  and  remote.  If 


320  GENETICS 

every  individual  regarded  the  germplasm  he  carries 
as  a  sacred  trust,  then  it  would  be  the  part  of  an 
awakened  eugenic  conscience  to  restrain  that  germ- 
plasm  when  it  is  known  to  be  defective  or,  when  it  is 
not  defective,  to  hand  it  on  to  posterity  with  at  least 
as  much  foresight  as  is  exercised  in  breeding  domestic 
animals  and  cultivated  plants. 

The  eugenic  conscience  is  in  need  of  development, 
and  it  is  only  when  it  becomes  thoroughly  aroused  in 
the  rank  and  file  of  society  as  well  as  among  the  lead- 
ers, that  a  permanent  and  increasing  betterment  of 
mankind  can  be  expected. 

5.     RESTRICTION  OF  UNDESIRABLE  GERMPLASM 

A  negative  way  to  bring  about  better  blood  in  the 
world  is  to  follow  the  clarion  call  of  Davenport  and 
"dry  up  the  streams  that  feed  the  torrent  of  defective 
and  degenerate  protoplasm." 

The  education  of  the  feeble-minded,  the  cure  of  the 
insane  and  the  reform  of  the  criminal  are  all  euthenic 
not  eugenic  means  of  relief.  Some  idea  of  the  extent 
of  the  drag  of  the  "submerged  tenth'*  upon  human  so- 
ciety may  be  gained  from  the  following  table,  the  data 
for  which  are  derived  from  the  U.  S.  census.1 

The  burden  of  the  three  undesirable  D's,  "defectives, 
dependents  and  delinquents,"  upon  human  society  is 
by  no  means  entirely  represented  in  the  dollar-column 
of  this  table.  Each  individual  recorded  is  a  human 
being,  the  member  of  some  family  and  community, 

1  Statistical  Directory  of  State  Institutions  for  the  Defective, 
Dependent  and  Delinquent  Classes.  Washington,  1919. 


HUMAN  CONSERVATION 


321 


STATE  INSTITUTIONS  FOR  DEFECTIVE,  DEPENDENT  AND 
DELINQUENT  CLASSES 


Institutions  for 

No.  of 
Inmates 
Jan.  1, 

No.  of 
Institu- 
tions 

Expenditures 
for  maintenance 
and  operation 

1916 

in  1915 

1.  Insane 

199,340 

147 

$36,312,662.20 

2.  Criminalistic 

95,985 

170 

21,244,892.00 

3.  Dependent 

45,373 

84 

9,675,932.37 

4.  Tuberculous 

7,187 

45 

3,539,454.95 

5.  Feeble-minded 

19,298 

27 

3,341,442.85 

6.  Deaf 

6,826 

33 

1,893,490.09 

7.  Epileptic 

6,097 

9 

1,345,821.57 

8.  Feeble-minded     and     epi- 

leptic 

6,984 

9 

1,285,500.05 

9.  Blind 

3,118 

28 

1,066,973.14 

10.  Blind  and  deaf 

2,233 

12 

615,468.41 

11.  Inebriate 

615 

3 

232,080.62 

12.  Deformed 

601 

4 

206,747.23 

13.  Criminalistic    and    depen- 

dent 

814 

1 

105,705.86 

14.  Feeble-minded,   blind   and 

deaf 

191 

1 

67,051.73 

15.  Blind,  deaf  and  dependent 

215 

1 

59,649.67 

16.  Leprous 

114 

2 

56,118.19 

TOTAL 

394,991 

576 

$81,048,990.93 

which  must  be  more  or  less  directly  borne  down  by  the 
unfortunate  one.  Moreover,  the  unfortunates  who  are 
in  institutions  are  but  a  small  percentage  of  the  total 
number  in  the  population  who  are  not  in  institutions. 
It  should  be  remembered  that  although  heredity  plays 
an  important  part  in  such  life-tragedies  it  is  not  en- 
tirely to  blame  for  these  depressing  data. 

The  restriction  of  undesirable  additions  to  our 
human  stock  may  be  partially  accomplished,  at  least 
in  America,  by  employing  the  following  agencies: — 
control  of  immigration;  more  discriminating  marriage 


GENETICS 

laws ;  a  quickened  eugenic  sentiment ;  sexual  segrega- 
tion of  defectives;  and  finally,  drastic  measures  of 
asexualization  when  necessary.  Providing  for  the  eu- 
genic elimination  of  defectives  is  as  truly  a  civic  duty 
as  administering  charity  to  them  after  they  are  born. 

A.    CONTROL    OF    IMMIGRATION 

The  enforcement  of  immigration  laws  tends  to  debar 
from  the  United  States  not  only  many  undesirable  indi- 
viduals, but  also  incidentally  to  keep  out  much  poten- 
tially bad  germplasm  that,  if  admitted,  might  play 
havoc  with  future  generations. 

For  example,  during  the  year  of  1908,  65  idiots, 
121  feeble-minded,  184  insane,  3741  paupers,  2900 
individuals  having  contagious  diseases,  53  tuberculous 
individuals,  136  criminals,  and  124  prostitutes  were 
caught  in  the  sieve  at  Ellis  Island  alone  and  turned 
back  from  this  country  by  the  immigration  officials 
in  spite  of  the  fact  that  an  average  of  only  8  cents  a 
head  was  expended  upon  inspection. 

These  7000  and  more  individuals  probably  were  the 
bearers  of  very  little  germplasm  that  we  are  nationally 
not  better  off  without. 

Eugenically,  the  weak  point  in  the  present  applica- 
tion of  immigration  laws  is  that  criteria  for  exclusion 
are  phenotypic  in  nature  rather  than  genotypic,  and 
consequently  much  bad  germplasm  comes  through  our 
gates  hidden  from  the  view  of  inspectors  because  the 
bearers  are  heterozygous,  wearing  a  cloak  of  desirabil- 
ity over  undesirable  traits. 


HUMAN  CONSERVATION 

It  is  not  enough  to  lift  the  eyelid  of  a  prospective 
parent  of  American  citizens  to  discover  whether  he  has 
some  kind  of  an  eye-disease  or  to  count  the  contents  of 
his  purse  to  see  if  he  can  pay  his  own  way.  The  offi- 
cial ought  to  know  if  eye-disease  runs  in  the  immi- 
grant's family  and  whether  he  comes  from  a  race  of 
people  which,  through  chronic  shiftlessness  or  lack  of 
initiative,  have  always  carried  light  purses. 

In  selecting  horses  for  a  stock-farm  an  expert  horse- 
man might  rely  to  a  considerable  extent  upon  his 
judgment  of  horseflesh  based  upon  inspection  alone,  but 
the  wise  breeder  does  more  than  take  the  chances  of 
an  ordinary  horse-trader.  He  wants  to  be  assured  of 
the  pedigree  of  his  prospective  stock.  It  is  to  be 
hoped  that  the  time  will  come  when  we,  as  a  nation, 
will  rise  above  the  hazardous  methods  of  the  horse 
trader  in  selecting  from  the  foreign  applicants  who 
knock  at  our  portals,  and  that  we  will  exercise  a  more 
fundamental  discrimination  than  such  a  haphazard 
method  affords,  by  demanding  a  knowledge  of  the  germ- 
plasm  of  these  candidates  for  citizenship,  as  displayed 
in  their  pedigrees. 

This  may  possibly  be  accomplished  by  having  trained 
inspectors  located  abroad  in  the  communities  from 
which  our  immigrants  come,  whose  duty  it  shall  be  to 
look  up  the  ancestry  of  prospective  applicants  and  to 
stamp  desirable  ones  with  approval. 

This  should  be  done  by  our  own  government  and  not 
by  labor  contractors  or  steamship  companies 'who  are 
not  actuated  by  any  » eugenic  considerations.  More- 
over, any  immigration  law  requiring  certificates  from 


324  GENETICS 

foreign  governments  would  seriously  interfere  with  our 
getting  many  desirable  foreigners  to  come  to  this 
country. 

The  national  expense  of  such  a  program  of  genea- 
logical inspection  would  be  far  less  than  the  mainte- 
nance of  introduced  defectives,  in  fact  it  would  greatly 
decrease  the  number  of  defectives  in  the  country.  At 
the  present  time  this  country  is  spending  over  one  hun- 
dred million  dollars  a  year  on  defectives  alone,  and 
each  year  sees  this  amount  increased. 

The  United  States  Department  of  Agriculture 
already  has  field  agents  scouring  every  land  for  desir- 
able animals  and  plants  to  introduce  into  this  country, 
as  well  as  stringent  laws  to  prevent  the  importation 
of  dangerous  weeds,  parasites,  and  organisms  of  vari- 
ous kinds.  Is  the  inspection  and  supervision  of  human 
blood  less  important? 

B.    MORE    DISCRIMINATING    MARRIAGE   LAWS 

Every  people,  including  even  the  more  primitive 
races,  make  customs  or  laws  that  tend  to  regulate 
marriage.  Of  these,  the  laws  which  relate  to  the 
eugenic  aspect  of  marriage  are  the  only  ones  that 
concern  us  in  this  connection.  "Marriage,"  says 
Davenport,  "can  be  looked  at  from  many  points  of 
view.  In  novels  as  the  climax  of  human  courtship; 
in  law  largely  as  two  lines  of  property  descent;  in 
society,  as  fixing  a  certain  status ;  but  in  eugenics, 
which  considers  its  biological  aspect,  marriage  is  an 
experiment  in  breeding." 


HUMAN  CONSERVATION  325 

Certain  of  the  United  States  have  laws  forbidding 
the  marriage  of  epileptics,  habitual  drunkards,  paupers, 
idiots,  the  insane,  feeble-minded,  and  those  afflicted 
with  venereal  diseases.  It  would  be  well  if  such  laws 
were  not  only  more  uniform  and  widespread,  but  also 
more  rigidly  enforced. 

The  fact  that  much  marriage  taboo  already  exists 
regardless  of  laws  which  effectually  hinder  or  prevent 
certain  kinds  of  undesirable  matings,  forms  a  basis  of 
hope  for  future  control. 

It  is  quite  true  that  marriage  laws  in  themselves  do 
not  necessarily  control  human  reproduction,  for  ille- 
gitimacy is  a  factor  that  must  always  be  reckoned  with ; 
nevertheless  such  laws  do  have  an  important  influence 
in  regulating  marriage  and  consequent  reproduction. 

Marriage  laws  may,  however,  sometimes  bring  about 
a  deplorable  result  eugenically,  as  in  the  case  of  forced 
marriage  of  sexual  offenders  in  order  to  legalize  the 
offense  and  "save  the  woman's  honor."  To  compel, 
under  the  guise  of  legality,  two  defective  streams  of 
germplasm  to  combine  repeatedly  and  thereby  result  in 
defective  offspring  just  because  the  unfortunate  event 
happened  once  illegitimately,  is  fundamentally  a  mis- 
take. Darwin  says :  "Except  in  the  case  of  man  him- 
self hardly  any  one  is  so  ignorant  as  to  allow  his  worst 
animals  to  breed." 


C.    AN    EDUCATED    SENTIMENT 

A  far  more  effective  means  of  restricting  bad  germ- 
plasm  than  placing  elaborate  marriage  laws  upon  our 


326  GENETICS 

statute-books  is  to  educate  public  sentiment  and  to 
foster  a  popular  eugenic  conscience,  in  the  absence 
of  which  the  safeguards  of  the  law  must  forever  be 
largely  without  avail  since  our  best  hope  lies  not  in 
compulsion  but  in  voluntary  effort. 

Such  a  sentiment  already  generally  exists  to  a  large 
extent  with  respect  to  incest,  and  the  marriage  of 
persons  as  noticeably  defective  as  idiots  or  those 
afflicted  with  insanity,  and  also  in  America  with  respect 
to  miscegenation,  but  a  cautious  and  intelligent  ex- 
amination of  the  more  obscure  defective  traits,  exhib- 
ited in  the  somatoplasms  of  the  various  members  of 
^families  in  question,  is  largely  an  ideal  of  the  future. 
Under  existing  conditions  non-eugenic  considerations 
such  as  wealth,  social  position,  etc.,  often  enter  into 
the  preliminary  negotiations  of  a  marriage  alliance, 
but  an  equally  unromantic  caution  with  reference  to 
the  physical,  moral,  and  mental  characters  that  make 
up  the  biological  heritage  of  contracting  parties  is 
less  usual. 

The  scientific  attitude  is  not  necessarily  opposed  to 
the  romantic  way  of  looking  at  things.  If  the  bandage 
across  the  eyes  of  blind  Cupid  is  allowed  to  slip  a  little 
in  so  important  and  far-reaching  an  operation  as 
"falling  in  love"  it  is  perhaps  just  as  well.  The  dia- 
logue in  "Two  Gentlemen  of  Verona"  between  Julia 
and  Lucetta  is  quite  to  the  point  where  the  eager  and 
curious  Julia  says  to  her  maid, — 

"But  say,  Lucetta,  now  we  are  alone, 
Woulds't  thou  counsel  me  to  fall  in  love  ?  " 


HUMAN  CONSERVATION 

and  the  canny  Lucetta  makes  reply, — 

"Aye,  Madam,  so  you  stumble  not  unheedfully." 

This  advice  is  simply  "organized  common  sense,"  and 
romance,  which  dispenses  with  this  balance-wheel,  al- 
though it  may  be  entertaining  and  always  exciting  at 
first,  is  sure  to  be  disappointing  in  the  end.  Marriages 
may  be  "made  in  heaven,"  but,  as  a  matter  of  fact, 
children  are  born  and  have  to  be  brought  up  on  earth, 
and  there  is  nothing  particularly  romantic  in  defective 
children  who  might  better  never  have  been  born.  It  fol- 
lows without  saying  that  it  will  be  much  easier  to  stamp 
out  bad  germplasm  when  an  educated  sentiment  be- 
comes common  among  all  people  everywhere. 

D.  SEGREGATION  OF  DEFECTIVES 

Persons  with  hereditary  defects,  such  as  epileptics, 
idiots,  and  certain  criminals,  who  become  wards  of 
the  state,  should  be  segregated  or  confined  in  comfort 
so  that  their  germplasm  may  not  escape  to  furnish 
additional  burdens  upon  society.  "We  have  become 
so  used  to  crime,  disease  and  degeneracy  that  we  take 
them  for  necessary  evils.  That  they  were,  in  the 
world's  ignorance,  is  granted.  That  they  must  remain 
so,  is  denied"  (Davenport). 

"The  great  horde  of  defectives  once  in  the  world 
have  the  right  to  live  and  enjoy  as  best  they  may 
whatever  freedom  is  compatible  with  the  lives  and  free- 
dom of  other  members  of  society,"  says  Kellicott,  "but 
society  has  a  right  to  protect  itself  against  repetitions 
of  hereditary  blunders." 


328  GENETICS 

There  is  one  grave  danger  connected  with  the  ad- 
ministration of  our  humane  and  commendable  philan- 
thropies for  the  unfortunate,  since  it  frequently  hap- 
pens that  defectives  are  kept  in  institutions  until  they 
are  sexually  mature  or  are  partly  self-supporting, 
when  they  are  liberated  only  to  add  to  the  burden  of 
society  by  reproducing  their  like. 

Furthermore,  if  defectives  of  the  same  sort  are  col- 
lected together  in  the  same  institutions,  unless  sexual 
segregation  is  strictly  maintained,  they  may  by  the 
very  circumstance  of  proximity  tend  to  reproduce 
their  kind  just  as  defectives  in  any  isolated  community 
tend  to  multiply.  There  is  much  misplaced  philan- 
thropy that  is  euthenic  but  not  eugenic.  The  tempo- 
rary troubles  of  the  individual  may  be  alleviated  only 
to  make  possible  a  future  addition  to  the  burden  of 
society. 

David  Starr  Jordan  cites  the  interesting  case  of 
cretinism  which  occurs  in  the  valley  of  Aosta  in 
northern  Italy,  to  prove  the  wisdom  of  the  sexual 
segregation  of  defectives.  Cretinism  is  an  hereditary 
defect  connected  with  an  abnormal  development  of 
the  thyroid  gland  which  results  in  a  peculiar  form  of 
idiocy  usually  associated  with  goitre. 

"In  the  city  of  Aosta  the  goitrous  cretin  has  been 
for  centuries  an  object  of  charity.  The  idiot  has  re- 
ceived generous  support,  while  the  poor  farmer  or 
laborer  with  brains  and  no  goitre  has  had  the  severest 
of  struggles.  In  the  competition  of  life  a  premium 
has  thus  been  placed  on  imbecility  and  disease.  The 
cretm  has  mated  with  cretm,  the  goitre  with  goitre, 


HUMAN  CONSERVATION  329 

and  charity  and  religion  have  presided  over  the  union. 
The  result  is  that  idiocy  is  multiplied  and  intensified. 
The  cretin  of  Aosta  has  been  developed  as  a  new  species 
of  man.  In  fair  weather  the  roads  about  the  city  are 
lined  with  these  awful  paupers — human  beings  with 
less  intelligence  than  a  goose,  with  less  decency  than 
the  pig." 

Whymper,  writing  in  1880,  further  observes:  "It  is 
strange  that  self-interest  does  not  lead  the  natives  of 
Aosta  to  place  their  cretins  under  such  restrictions  as 
would  prevent  their  illicit  intercourse;  and  it  is  still 
more  surprising  to  find  the  Catholic  Church  actually 
legalizing  their  marriage.  There  is  something  horribly 
grotesque  in  the  idea  of  solemnizing  the  union  of  a 
brace  of  idiots,  and,  since  it  is  well  known  that  the  dis- 
ease is  hereditary  and  develops  in  successive  genera- 
tions the  fact  that  such  marriages  are  sanctioned  is 
scandalous  and  infamous." 

Since  1890  the  cretins  have  been  sexually  segregated, 
and  in  1910  Jordan  reported  that  they  were  nearly 
all  gone. 

E.    DRASTIC    MEASURES 

A  fifth  method  of  restricting  undesirable  germplasm 
in  the  case  of  confirmed  criminals,  idiots,  imbeciles,  and 
rapists  may  be  mentioned,  namely,  the  extreme  treat- 
ment of  either  asexualization  or  vasectomy.  The  lat- 
ter is  a  minor  operation  confined  to  the  male  which 
occupies  only  a  few  moments  and  requires  at  most  only 
the  application  of  a  local  anaesthetic,  such  as  cocaine. 
There  are  probably  no  disturbing  or  even  inconvenient 


330  GENETICS 

after  effects  from  this  operation.  It  consists  in  remov- 
ing a  small  section  of  each  sperm  duct  and  is  entirely 
effectual  in  preventing  subsequent  parenthood. 

In  the  female  the  corresponding  operation,  which 
consists  in  removing  a  portion  of  each  Fallopian  tube, 
is  much  more  severe,  but  not  impracticable  or  dan- 
gerous. 

According  to  Laughlin  who  has  carefully  collected 
data  on  the  subject,  in  ten  of  the  fifteen  states  which 
have  enacted  eugenical  sterilization  statutes  the  law  is 
still  (1921)  on  the  statute  books,  unattacked  b} 
courts  and  so  still  available  for  use.  From  the  be- 
ginning of  legal  sterilization  in  the  United  States  in 
1907  until  January  1st,  1921,  a  total  of  3233  caco- 
genic  persons  have  been  made  sexually  sterile  under 
the  several  statutes. 

Laughlin  goes  on  to  point  out  that  "the  nature  of 
administrative  machinery,  which  will  work  and  which 
will  fail,  is,  from  the  experiments  already  made,  fairly 
well  known,  so  that  if  the  principle  of  eugenical  sterili- 
zation has  public  support,  practically  any  state  legis- 
lature can,  if  it  chooses,  enact  a  well-functioning  law." 

The  possibility  of  the  abuse  of  sterilization  if  legal- 
ized is,  however,  so  great  that  this  extreme  method  of 
last  resort  will  be  for  a  long  time  doubtless  of  very 
questionable  application. 

6.  THE  CONSERVATION  OF  DESIRABLE  GERMPLASM 

The  eugenic  ideal  may  be  approached  not  only  nega- 
tively by  the  restriction  of  undesirable  germplasm,  but 


HUMAN  CONSERVATION  331 

also  positively  by  the  conservation  of  desirable  germ- 
plasm. 

The  various  ways  in  which  this  improvement  of 
society  may  be  brought  about  are: 

A.    BY     ENLARGING     INDIVIDUAL     OPPORTUNITY 

Much  good  human  germplasm  goes  to  waste  through 
ineffectiveness  on  account  of  unfavorable  environment 
or  lack  of  a  suitable  opportunity  to  develop. 

Every  agency  which  contributes  toward  increasing 
the  opportunity  of  the  individual  to  attain  to  a  better 
development  of  his  latent  possibilities  is  in  harmony 
with  a  thoroughly  positive  eugenic  practice.  Thus 
better  schools,  better  homes,  better  living  conditions, 
in  short,  all  euthenic  endeavor,  directly  serves  the 
eugenic  ideal  by  making  the  best  out  of  whatever  ger- 
minal equipment  is  present  in  man. 

B.    BY   PREVENTING   GERMINAL   WASTE 

Much  good  protoplasm  fails  to  find  expression  in 
the  form  of  offspring  because  one  or  the  other  of  pos- 
sible parents  is  cut  off  either  by  preventable  death  or 
by  social  hindrances.  To  avoid  such  calamities  is  a 
part  of  the  positive  program  of  eugenics. 

a.  Preventable  Death 

War,  from  the  eugenic  point  of  view,  is  the  height 
of  folly,  since  presumably  the  brave  and  the  physically 
fit  march  away  to  fight,  while  in  general  the  unqualified 


333  GENETICS 

stay  at  home  to  reproduce  the  next  generation.  When 
a  soldier  dies  on  the  battlefield  or  in  the  hospital,  it 
is  not  alone  a  brave  man  who  is  cut  off,  but  it  is  the 
termination  of  a  probably  desirable  strain  of  germ- 
plasm. 

David  Starr  Jordan  has  presented  this  matter  very 
clearly.  He  points  out  that  the  "man  with  a  hoe" 
among  the  European  peasantry  is  not  the  result  of 
centuries  of  oppression,  as  he  has  been  pictured,  but 
rather  the  dull  progeny  resulting  from  generations  of 
the  unfit  who  were  left  behind  when  the  fit  went  off  to 
war  never  to  return. 

Benjamin  Franklin,  with  characteristic  wisdom,  sums 
up  the  situation  in  the  following  epigram:  "Wars  are 
not  paid  for  in  war  time;  the  bill  comes  later." 

b.  Social  Hindrances 

There  are  many  conditions  of  modern  society  which 
act  non-eugenically. 

For  instance,  the  increasing  demands  of  profes- 
sional life  prolong  the  period  necessary  for  prepara- 
tion, which,  with  the  "cost  of  high  living,"  tends 
toward  late  marriage.  In  this  way  much  of  the  best 
gennplasm  is  very  often  withheld  from  circulation 
until  it  is  too  late  to  be  effective  in  providing  for  the 
succeeding  generation. 

Certain  occupations  such  as  school-teaching  and 
nursing  by  women  are  filled  by  the  best  blood  obtainable, 
yet  this  blood  is  denied  a  direct  part  in  molding  pos- 
terity, since  marriage  is  frequently  either  forbidden  or 


HUMAN  CONSERVATION  333 

regarded  as  a  serious  handicap  in  such  lines  of  work. 
Advertisements  concerning  "unincumbered  help"  and 
"childless  apartments"  tell  their  own  deplorable  tale. 
One  of  the  darkest  features  of  the  dark  ages  from 
a  eugenic  standpoint  was  the  enforced  celibacy  of  the 
priesthood,  since  this  resulted,  as  a  rule,  in  withdrawing 
into  monasteries  and  nunneries  much  of  the  best  blood 
of  the  times,  and  this  uneugenic  custom  still  obtains  in 
many  quarters  to-day. 


C.    BY   SUBSIDIZING   THE   FIT 

It  is  possible  that  if  some  of  the  philanthropic  en- 
deavor now  directed  toward  alleviating  the  condition 
of  the  unfit  should  be  directed  to  enlarging  the  oppor- 
tunity of  the  fit,  greater  good  would  result  in  the 
end.  In  breeding  animals  and  plants  the  most  notable 
advances  have  been  made  by  isolating  and  developing 
the  best,  rather  than  by  attempting  to  raise  the  stand- 
ard of  mediocrity  through  the  elimination  of  the  worst. 

One  leader  is  worth  a  score  of  followers  in  any  com- 
munity, and  the  science  of  genetics  surely  gives  to  edu- 
cators the  hint  that  it  is  wiser  to  cultivate  the  excep- 
tional pupil  who  is  often  left  to  take  care  of  himself 
than  to  expend  all  the  energies  of  the  instructor  in 
forcing  the  indifferent  or  ordinary  one  up  to  a  passing 
standard.  The  campaign  for  human  betterment  in  the 
long  run  must  do  more  than  avoid  mistakes.  It  must 
become  aggressive  and  take  advantage  of  those  human 
mutations  or  combinations  of  traits  which  appear  in  the 
exceptionally  endowed. 


334  GENETICS 

7.     WHO  SHALL  SIT  IN  JUDGMENT? 

In  the  practical  application  of  a  program  of  eu- 
genics there  are  many  difficulties,  for  who  is  qualified 
to  sit  in  judgment  and  separate  the  fit  from  the  unfit? 

There  are  certain  strongly  marked  characteristics 
in  mankind  which  are  plainly  good  or  bad,  but  the 
principle  of  the  independence  of  unit  characters  dem- 
onstrates that  no  person  is  wholly  good  or  wholly  bad. 
Shall  we  then  throw  away  the  whole  bundle  of  sticks 
because  it  contains  a  few  poor  or  crooked  ones?  Is  it 
wise  to  burn  the  barn  in  order  to  kill  the  rats? 

The  list  of  weakling  babies,  for  instance,  who  were 
apparently  physically  unfit  and  hardly  worth  raising 
upon  first  judgment,  but  who  afterwards  became  power- 
ful factors  in  the  world's  progress,  is  a  notable  one  and 
includes  the  names  of  Calvin,  Newton,  Heine,  Voltaire, 
Herbert  Spencer  and  Robert  Louis  Stevenson. 

Dr.  C.  V.  Chapin  recently  said  with  reference  to 
the  eugenic  regulation  of  marriage  by  physician's 
certificate:  "The  causes  of  heredity  are  many  and 
very  conflicting.  The  subject  is  a  difficult  one,  and 
I  for  one  would  hesitate  to  say,  in  a  great  many  cases 
where  I  have  a  pretty  good  knowledge  of  the  family, 
where  marriage  would,  or  would  not,  be  desirable." 

Desirability  and  undesirability  must  always  be  re- 
garded as  relative  terms  more  or  less  undefinable.  In 
attempting  to  define  them,  it  makes  a  great  difference 
whether  the  interested  party  holds  to  a  puritan  or 
a  cavalier  standard.  To  show  how  far  human  judg- 
ment may  err  as  well  as  how  radically  human  opinion 


HUMAN  CONSERVATION  335 

changes,  there  were  in  England,  as  recently  as  1819, 
233  crimes  punishable  by  death  according  to  law. 

One  needs  only  to  recall  the  days  of  the  Spanish 
Inquisition  or  of  the  Salem  witchcraft  persecution  to 
realize  what  fearful  blunders  human  judgment  is 
capable  of,  but  it  is  unlikely  that  the  world  will  ever 
see  another  great  religious  inquisition,  or  that  in  ap- 
plying to  man  the  newly  found  laws  of  heredity  there 
will  ever  be  undertaken  an  equally  deplorable  eugenic 
inquisition. 

It  is  quite  apparent,  finally,  that  although  great 
caution  and  broadness  of  vision  must  be  exercised  in 
bringing  about  the  fulfillment  of  the  highest  eugenic 
ideals,  nevertheless  in  this  direction  lies  the  future 
path  of  human  achievement. 

8.  EUGENICS,  NOT  "BLUEGENICS" 

Eugenics  has  been  called  the  "dismal  science"  by  ro- 
mantic people  who  chafe  under  the  restrictions  of 
common  sense,  and  by  conscientious  individuals  who 
are  depressed  by  the  appalling  hereditary  blunders 
made  by  mankind,  but,  as  a  matter  of  fact,  eugenics 
presents  the  brightest  hope  for  the  future  of  humanity. 
Some  of  the  unattractiveness  of  the  eugenical  program 
lies  in  the  fact  that  it  calls  for  results  in  the  distant 
future  in  which  there  can  be  little  or  no  personal  par- 
ticipation, and  often  at  the  expense  of  present  day 
comforts.  It  is  a  lofty  ideal  of  altruism  and  patriot- 
ism, and  in  the  words  of  Major  Leonard  Darwin,  "an 
ideal  to  be  followed  like  a  flag  in  battle  without  thought 
of  personal  gain" 


336  GENETICS 

9.  THE  MORAL 

Race-preservation,  not  self-preservation  is  the  first 
law  of  nature.  Because  the  laws  of  heredity  work  re- 
lentlessly within  predetermined  limits  is  iio  reason  for 
branding  eugenics  with  the  mark  of  a  fatalistic  philo- 
sophy that  would  avoid  personal  responsibility.  The 
Florida  orange-grower  who  uses  his  intelligence  and 
plants  frost-resisting  varieties  to  replace  those  over- 
taken by  frost  does  not  blame  fate  for  his  losses.  It 
is  never  fatalistic  to  seek  to  find  out  the  true  determin- 
ing causes  of  a  disaster  and  to  apply  the  obvious 
remedy.  As  Osborn  has  said: — "To  know  the  worst 
as  well  as  the  best  in  heredity;  to  preserve  and  select 
the  best, — these  are  the  most  ^essential  forces  in  the 
future  evolution  of  human  society." 

Our  hereditary  endowment  may  be  something  given 
us  without  our  consent  and  connivance  and  the  accident 
of  our  birth  may  determine  very  largely  the  environment 
in  which  we  must  work  out  our  salvation  but  there 
lies  a  sleeping  giant  of  possibility  in  everyone,  and, 
whether  we  have  one  talent  or  five  or  ten,  the  individual 
response  we  make  is  our  own  and  we  alone  are  respon- 
sible for  it. 

Finally,  to  quote  the  wise^words  of  Huxley, —  "To 
learn  what  is  true  in  order  to  do  what  is  right"  is  the 
summing  up  of  the  whole  duty  of  man,  for  all  VhoTtlre 
not  able  to  satisfy  their  mental  hunger  with  the  east 
wind  of  authority." 


BIBLIOGRAPHY 

A  few  recent  works  of  a  general  nature  are  listed  below. 
Several  of  these  books,  particularly  those  that  are  starred,  con- 
tain bibliographies  of  technical  papers  and  other  original  sources 
of  information. 

*Babcock,  E.  B.,  and  R.  E.  Clausen,  1918.  "Genetics  in  Relation 
to  Agriculture."  New  York. 

Bateson,  W.  1894.  "Materials  for  the  Study  of  Variation." 
London. 

1913.    "Problems  of  Genetics."    Yale  Univ.  Press. 
1913.    Mendel's  "Principles  of  Heredity."     New  York. 

Baur,  E.  1914.  "Einfiihrung  in  die  Experimentelle  Vererbungs- 
lehre."  2te  Auf.  Berlin, 

Castle,  W.  E.  1911.  "Heredity  in  Relation  to  Evolution  and 
Animal  Breeding."  New  York. 

*  Castle,  W.  E.  1920.  "Genetics  and  Eugenics."  2nd  Ed.  Har- 
vard Univ.  Press. 

Castle,  W.  E.;  J.  M.  Coulter;  C.  B.  Davenport;  E.  M.  East; 
W.  L.  Tower.  1912.  "Heredity  and  Eugenics."  Chicago. 

Conklin,  E.  G.  1915.  "Heredity  and  Environment."  Princeton 
Univ.  Press. 

Correns,  C.     1912.     "Die  neuen  Vererbungsgesetze."     Berlin. 

Coulter,  J.  M.  1914.  "Fundamentals  of  Plant-Breeding."  New 
York  and  Chicago. 

Darbishire,  A.  D.  1912.  "Breeding  and  the  Mendelian  Discov- 
ery." London. 

Darwin,,  C.  1859.  "The  Origin  of  Species  by  Means  of  Natural 
Selection,  or  the  Preservation  of  Favored  Races  in  the  Struggle 
for  Life."  New  York. 

1868.     "The  Variation  of  Animals  and  Plants  under  Domesti- 
cation."    2nd  Ed.     New  York. 

Davenport,  C.  B.  1911.  "Heredity  in  Relation  to  Eugenics." 
New  York. 

337 


338  GENETICS 

Doncaster,  L.    1911.    "Heredity  in  the  Light  of  Recent  Research." 
Cambridge  Univ.  Press. 
1914.     "The  Determination  of  Sex."    Cambridge  Univ.  Press. 

East,  E.  M.     1907.     "The  Relation  of  Certain  Biological  Princi- 
ples to  Plant  Breeding."     Bull.   158  Conn.   Agric.   Sta. 

East,  E.  M.  and  D.  F.  Jones.     1919.     "Inbreeding  and  Outbreed- 
ing."     Philadelphia. 

Galton,  F.    1883.    "Inquiries  into  Human  Faculty."    New  York. 
1889.     "Natural  Inheritance."     London. 
1892.     "Hereditary  Genius."    London. 

Gates,  R.  R.     1915.     "The  Mutation  Factor  in  Evolution."     Lon- 
don. 

Goddard,  H.  H.  1912.  "The  Kallikak  Family."  New  York. 
1914.  "Feeble-mindedness ;  Its  Causes  and  Consequences." 
New  York. 

Godlewski,    E.      1909.      "Das    Vererbungsproblem   im   Lichte   der 
Entwicklungsmechanik."     Leipzig. 

Goldschmidt,   R.     1911.     "Einfiihrung  in   die  Vererbungswissen- 
schaft."     Leipzig. 

Guyer,  M.  F.     1916.     "Being  Well  Born."     Indianapolis. 

Haecker,    V.       1912.      "Allgemeine     Vererbungslehre."       Braun- 
schweig. 

*Hall,  Gertrude  E.    1913.    "Eugenics  and  Social  Welfare."    Bull. 
No.  3,  State  Board  of  Charities,  Albany,  N.  Y. 

Hays,  W.  M.     1902-4.     "Breeding  Plants  and  Animals."     Minne- 
apolis. 

Johannsen,  W.     1913.     "Elemente  der  exakten  Erblichkeitslehre." 
2te  Auf.    Jena. 

Kellicott,  W.  E.     1911.     "The  Social  Direction  of  Human  Evolu- 
tion."    New  York. 
Kronacher,     C.       1912.       "Grundziige     der     Zuchtungsbiologie." 

Berlin. 
Lang,    A.      1914.      "Die    experimentelle    Vererbungslehre    in    der 

Zoologie  seit  1900."    Jena. 

Lock,  R.  H.     1911.     "Variation,  Heredity  and  Evolution."     Lon- 
don. 

Lotsy,  J.  P.     1906-1908.    "Vorlesungen  iiber  Descendenztheorien." 
Jena. 
1916.     "Evolution  by  Means  of  Hybridization."     The  Hague. 


BIBLIOGRAPHY  339 

Mendel,  G.     1865.     "Versuche  iiber  Pflanzen-hybriden."     Verb.  d. 
Naturf.  Vereins  in  Briinn.     Translated  in  Castle,  1920. 

Montgomery,  T.  H.     1906.     "The  Analysis  of  Racial  Descent  in 
Animals."    New  York. 

Morgan,  T.    H.     1913.     "Heredity  and  Sex."     New  York. 

1916.     "A   Critique  of  the  Theory  of  Evolution."     Princeton 
Univ.  Press. 

*Morgan,  T.  H.    1919.    "The  Physical  Basis  of  Heredity."    Phila- 
delphia. 

Morgan,  T.  H.,  A.  H.  Sturtevant,  H.  J.  Muller,  and  C.  B.  Bridges. 
1915.     "The  Mechanism  of  Mendelian  Heredity."     New  York. 

Newman,   H.   E.     1921.     "Readings   in   Evolution,   Genetics   and 
Eugenics."     Univ.  of  Chicago  Press.     Chicago. 

Pearl,  R.    1915.    "Modes  of  Research  in  Genetics."    New  York. 

Plate,  L.     1913.     "Vererbungslehre."  Leipzig. 

Popenoe,   P.    and    R.    H.    Johnson.      1918.      "Applied   Eugenics." 
New  York. 

Punnett,  R.  C.     1919.     "Mendelism."     Fifth  Ed.     New  York. 

Reid,  Archdall     1905.    "The  Principles  of  Heredity."     London. 

Reid,  G.  A.    1910.     "The  Laws  of  Heredity."     London. 

Rignano,  E.     1911.     "Upon  the  Inheritance  of  Acquired  Charac- 
ters."   Translated  by  B.  C.  H.  Harvey.     Chicago. 

Saleeby,  C.  W.     1909.     "Parenthood  and  Race  Culture;  An  Out- 
line of  Eugenics."     New  York. 
1914.     "The  Progress  of  Eugenics."    New  York  and  London. 

Schallmayer,  W.     1910.     "Vererbung  und  Auslese  im  Lebenslauf 
der  Volker."     Jena. 

Schneider,  K.  G.     1911.     "Emfiihrung  in  die  Descendenztheorie." 
Jena. 

Schuster,  E.     1912.     "Eugenics."     London. 

Semon,  R.    1912.    "Das  Problem  der  Vererbungslehre  erworbener 
Eigenschaften."     Leipzig. 

Sharp,  L.  W.     1921.     "An  Introduction  to  Cytology."    New  York. 

Thomson,  J.  A.     1908.     "Heredity."     London. 

Watson,  J.  A.  S.     1912.     "Heredity."     New  York. 

Weismann,  A.     1904.     "The  Evolution  Theory."     London. 

1893.  "The  Germplasm.  A  Theory  of  Heredity."  English 
translation  by  W.  N.  Parker  and  Harriet  Ronnfeldt.  New 
York. 


340  GENETICS 

Woods,  F    A.     1906.     "Mental  and  Moral  Heredity  in  Royalty." 

New  York. 
de   Vries,    H.      1905.     "Species   and   Varieties,   their    Origin   by 

Mutation."     Chicago. 

1901-3.     "Die  Mutationstheorie."     Leipzig. 
Ziegler,  H.  E.     1918.     "Die  Vererbungslehre  in  der  Biologic  und 

in  der  Soziologie."    Jena. 


INDEX 


Aberrations,  chromosomal,  58 
Ability,  artistic,  305 

literary,  305 

musical,  305 
Abnormal  fertilization,  230 

ACKERT,   137 

Acquired  characters,  62,  91 

Adam  and  Eve,  308 

AGAR,  137 

Agouti,  154,  161 

Aggregate  mutation,  51 

Albinism,  52,  199,  235,  306 

Albino  animals,  49,  155,  161 

Alcoholism,  80,  198,  311 

Allelomorphs,  99,    107,  149 

Alpine  flora,  77 

Alternative  genes,  149,  150,  160 

inheritance,  94 
ALTENBERG,  249 
Amblystoma,  78,  102 
Ammonites,  56 
Amphimixis,  36,  69 
Anaphase,    220,    221,    226,    267, 

271,  273 

Ancon  sheep,  49 
Andalusian  fowl,  169,  174 
Angora,  102 
Annelids,  293 
Ant-eater,  spiny,  198 
Anti-body,  89 
Aphids,  137,  272,  273,  275 
Appendix,  vermiform,  21,  197 
Apples,  greening,  11 
Arabs,  208 
Arcella,  137 

Architecture  of  germplasm,  233 
ARISTOTLE,  316 

Arithmetical  mean,  23,  26,  42 
Armadillo,  277 
Arrested  development,  197 
Artistic  ability,  305 


Ascaris,  14,  218 
Ascidian,  259 
Asexualization,  329 
Asexual  spores,  12 
Asthenic,  317 
Atavism,   194 
Autosomes,   266 
Average  deviation,  26,  27 
Axolotl,  78 
Azaleas,  double,  47 

BABCOCK,  51,  57,  85 

Babies,  weakling,  334 

Bacteria,  80,  136,  138 

BALLS,  102 

BALTZER,  293,  294 

Banana  fly  (see  Drosophila) 

Banded  shell,  102 

BANTA,  137,  142,  291 

BARBER,  136 

Bare  neck  in  poultry,  50 

Bar-eye,  53,  137 

Barley,  102,  136,  202 

Barring,  283,  284 

BATESON,  3,  22,  37,  39,  46,  50, 

95,   96,   100,   102,   106,   151, 

152,  161,  174,  176,  197,  235, 

249 

Battle  scars,  75 
BAUR,  38,  102,  165,  174,  249 
Beans,  123,  124,  125,  128,  136, 

249 

Beardlessness,  102 
BEECHER,  56 
Beech  leaves,  25,  31,  33 

purple,  47 
Beetles,  potato,  33,  36,  137,  143, 

144 

Begonia,  10,  11 
BEINHART,  55 
Belemnites,  56 


341 


342 


INDEX 


Belgian  hare,  177,  187 

BELL,  318 

BELLING,  59 

Bertillon  system,  19 

BIFFEN,  102,  296,  297 

Bimodal  polygons,  29 

Biometry,  23,  254 

Birds,  199,  270,  283,  288 

Birthmarks,  83 

Black-eyed  susan,  174 

BLAKESLEE,  59,  173 

BLARINGHEM,  76 

Blending  inheritance,  93,  168 

Blind,  309,  321 

Bluegenics,  335 

Bob-tail,  172 

Bonellia,  293 

Booted  poultry,  176 

BOVERI,   14,  218,  219,  220,  226, 

231,  249,  291 
Brachydactyly,  102,  172 
Branched  habit,  102 
Breeding,  experimental,  254 

pedigree,  200,  203 
BHEGGAR,  249 

BRIDGES,  53,  59,   145,  243,  246, 
248,  269,  270,  285,  287,  292 
Bristles,  dichaet,  137 

thoracic,  137,  143,  145 
BROOKS,  63 

Brown-eyed  yellow,  157 
Bryozoans,  11 
Bud  variations,  55 
BUBBANK,  210,  211 

Cabbage  butterflies,  51 

Cacaesthenic,  317 

Calf,  two-headed,  22 

CALKINS,  136 

Callosities,  78 

CALVIN,  334 

Canaries,  102 

Capaella,  77 

Carnation,  47 

Carnegie  Institution,  318 

Carriers,  282 

CASTLE,  7,  85,  86,  96,  102,  115, 
117,  118,  137,  146,  154,  157, 
158,  159,  161,  162,  166,  177, 
178,  189,  207,  249 


Castration,  289 
Cats,  235 

tailless,  49,  172 
Cattle,  102,  205,  288 

color,  170 

hairless,  50 

hornless,  49,  172 
Causes  of  mutation,  57,  60 

variation,  34 
Celandine,  47,  172 
Celibacy,  333 
Cell  differentiation,  72 

germ,  221,  55 

polar,  54,  224,  267,  271,  273 

sex,  224 

theory,  215 

typical,  216 

wall,  216 
Centropyxis,  137 
Centrosome,  216,  218 
Cereals,  204 
Cerebral  hernia,  50 
CHAPIN,  334 
Characters,  acquired,  62 

congenital,  68 

dominant,  195 

germinal,  67 

individual  unit,  117 

mental,  303 

moral,  303 

prenatal,  68 

recessive,  119,  196 

secondary  sexual,  287 

somatic,  67 
Chelidonium,  47 
Chiasmatype  theory,  243 
Chimaera,  55 

Chinese  women,  feet  of,  15 
Chromatin,  216,  217 
Chromosomal  aberrations,  58 
Chromosomes,  217,  226,  243,  248 

cycle,  273 
Chromosome,  extra,  229 

maps,  247,  248 

number,  218 

sex,  245,  265 

theory,  229,  254 

x,  266,  268,  27?,  277,  280,  285 

y,  268,  279 

z,  283,  284 


INDEX 


343 


Chrysanthemum,  31,  32 

Cinderella,  145,  233 

Circumcision,  75 

CLAUSEN,  57,  85 

Clones,     130,     134,     135,     136, 

138 

Clovers,  205 
Cirripede,  290 
Coat  pattern,  137 
COBB,  162,  177,  178 
COCKEREL,  48 
Cocoon,  102 

Coefficient  of  variability,  26 
Coelenterates,  292 
CCLE,  249 
Coleus,  136 
C 'olios,  51 
Color  blindness,  280,  282 

eye,  17,  20,  53,  102,  170,  195, 
235,  305 

gene,  152,  161 

roan,  170 

skin,  190 
Comb,  50 

Combinations,  38,  39,  51 
Complementary  genes,  149,  150, 

151 

Congenital  characters,  68 
CONKLIN,  63,  76,  151,  165,  193, 

253,  256,  258,  259,  260 
Connecting  links,  2 
Consanguineous   marriage,   208, 

312 

Conscience,  eugenic,  320 
Conservation  of  germplasm,  330 

human,  314 
Continuity,  13,  251 
Constants,  25,  26 
Convergent  variation,  198 
CORRENS,  96,  100,  102,  169 
Cotton,  52,  102 
Coupling,  235 
Cousin  marriage,  209,  307 
Crab,  290 

Crepidula,  293,  294 
Crested  head,  102 
Cretinism,  328 
Criminals,    299,    304,    309,    311, 

317,  320,  321,  322,  329 
Crippled  toes,  75 


Criss-cross      inheritance,      279, 

280,  281' 
Crosses,  homozygous,   132,   133, 

134,  136,  142 
negro-white,  190 
Cross-over,   238,  240,  241,  242, 

243 

Crustacea,  35,  290,  293 
Ctenophores,  292 
CUENOT,  96,  155,  161 
Cumulative  genes,  149,  150 
Cupid,  313,  326 
Curly  hair,  111 
Curves,  skew,  31 
Cycle,  chromosome,  273 
life,  56 
sexual,  272 
Cytoplasm,  216,  257 
Cytoplasmic  inheritance,  257 

Daisy,  31,  32 

DANIELSON,  190,  191,  192 

DANTE,  316 

Daphnids,  137,  142,  274,  291, 
292 

DARBISHIRE,  100,  102,  173 

DARWIN,  2,  3,  20,  23,  24,  34,  37, 
40,  41,  51,  61,  65,  70,  73,  95, 
96,,  105,  151,  166,  199,  205, 
206,  223,  297,  325 

DARWIN,  Maj.  L.,  335. 

Datura,  59,  174. 

DAVENPORT,  28,  50,  96,  102,  111, 
171,  172,  175,  176,  190,  191, 
192,  196,  200,  300,  304,  306, 
309,  312,  318,  324,  327 

DAVIS,  46    . 

Deaf,  309,  321 

Deafness,  235,  306,  318 

Death,  10,  56 

Deaths,  preventable,  331 

Deer,  207 

Deer-mouse,  52 

Defectives,  segregation  of,  327 

Defective  teeth,  235 

Defects,  control  of,  309 
hereditary,  306 

Deformed,  317,  321 
trees,  76 

Degressive  species,  46 


344 


INDEX 


Department  of  Agriculture,  324 

Dependent,  321 

Desirable  germplasm,  330 

Determination  of  sex,  264,  267 

DETTO,  70 

Development,  arrested,  197 

rate  of,  262 
Deviation,  standard,  26,  27 

average,  26,  27 

DEVRIES,  39,  41,  42,  43,  44,  45, 
46,  52,  56,  96,  102,  124,  257 
Diathetic,  317 
Differentiation,  cell,  219 

somatic,  255,  256,  258 
Difilugia,  136,  141 
Dihybrid,  107 
Diluting  gene,  156 
Dimorphism  in  sex,  287 
Dinosaurs,  56 
Disease,  80 

Dissolution  of  hybrids,  46 
Disuse,  effects  of,  78 
Dogs,  hairless,  50 

tailless,  49 
Dominance,  119,  168 

delayed,  170 

incomplete,  174 

imperfect,  168 

reversed,  171 

Dominant,  97,  99,  196,  212 
DONCASTER,  270,  284 
Double  flowers,  47 
Doves,  272,  295 
Drastic  measures,  329 
DRINKARD,  102 

Drosophila,  51,  52,  53,  101,  137, 
143,  145,  165,  171,  179,  207, 
233,  234,  235,  237,  239,  240, 
242,  245,  246,  247,  248,  249, 
269,  270,  278,  281,  285,  287, 
291 

DROWNE,  11 
Drugs,  effect  of,  82 
DRYDEN,  201 
Ducks,  289 

DUERDEN,   52 

DUGDALE,  299,  300 
DUNK,  249,  260 
Duplex,  106,  195 
Duplicate  genes,  190 


DURHAM,  156,  161 

Dwarf  peas,  98,  99,  100,  105,  107 

Dyads,  266 

DZIERZON,  276 

EAST,  136,  186,  187,  214 
Ears  of  rabbits,  177,  187 
Echidna,  198 
Echinoderms,  231 
Echinus,  231 

EDWARDS,  JONATHAN,  301 
Effects  of  drugs,  82 

use  and  disuse,  78 
Egg,  221 

abortive,  224 

ephippial,  275 

fertilized,  15,  55,  58,  80,  223 

frog's,  259 

human,  228 

laying,  145 

mature,  224 

parthenogenetic,  273,  275 

winter,  273,  274 
EHRLICH,  82 
EIMER,  21 

EI.DERTON,  306,  307 
Elementary  species,  41 
Ellis  Island,  322 
Embryology,  251,  254 
EMERSON,  48,  249 
Emperor  K'ang  Hsi,  204 
Endocrine  factors,  261 
Endocrinology,  262 
Environment,    46,    60,    76,   296, 

304,  315 

Environmental  factors,  261 
Enzymatic  pangenes,  257 
Ephippial  eggs,  275 
Epigenesis,  252 
Epileptic,  317,  321,  325 
Erinaceus,  198 
Erithyzon,  198 

ESTERBROOK,    300,    301 

Eugenics,  315,  317,  335 
Eugenic  conscience,  320 

Record  Office,  315 
Euthenics,  315 

Evening  primrose,  42,  52,  53,  56 
EWINO,  137 
Experimental  breeding,  254 


INDEX 


345 


Extension  gene,  157 
External  factors,  261,  275 
Extracted  recessives,  310 
Extra  chromosomes,  229 

toes  in  poultry,  50,  171 
Eye  color,  17,  20,  53,  102,  170, 
195,  235,  305 

Factor  hypothesis,  148 

endocrine,  261 

environmental,  254 

external,  261 

modifying,  297 

mutation,  58,  60 
Falling  in  love,  326 
False  reversion,  197 
FARRABEE,  102 
FAY,  306 
Feathers,  50 
Fecundity,  137 

Feeble-mindedness,  197,  302, 
303,  309,  311,  316,  320,  321, 
322,  325 

Feet  of  Chinese  women,  75 
Feral  animals,  198 
Fertilization,  224,  225,  226,  268 

abnormal,  230 

self,  97,  134 

Fertilized  eggs,  55,  58,  223 
Fever,  Texas,  82 
Fibers,  mantle,  220 
Filial  generation,  103 
FISH,  118,  145 
Fission,  10,  11,  87 
Fission  rate,  136,  137 
Flatworms,  293 
Flavism,  199 
Fluctuation,  42,  129 
Four-leaved  clover,  20 
Four-o'-clock,  106,  169,  174 
Fowls,  89,   171,  289 

blue  Andalusian,  169,  174 

jungle,  60,  196 

rumpless,  172 
FRANKLIN,  322 
Freaks,  51 
Free  martin,  288 
Frequency  polygon,  29 
Frog's  eggs,  259 
FEUWIRTH,  136 


GAGE,   71 

GALTON,  23,  24,  65,  93,  94,  121, 
122,  130,  155,  199,  302,  318 

Laboratory  of  Eugenics  318 
Gametes,  223 
Gametic  mutation,  53 
Gametogenesis,  254 
Garlic,  136 
GATES,  46,  59 
Gelastocoris,  269 
Gemmules,  11,  73 
Generation,  filial,  103 

parental,  103 

spontaneous,  9 
Gene,   149 

allelomorphic,  149 

alternative,  149,  150, 1«0 

arrangement  of,  245 

color,  152,  161 

complementary,  149,  150,  151 

constant,  160 

cumulative,  149,  150 

diluting,  156 

duplicate,  190 

extension,  157 

kinds  of,  149 

lethal,  149,  151,  162 

localization  of,  246 

intensifying,   156 

modifying,  146,  149,  150 

pattern,  154,  161 

pigment,  152 

plural,  149 

restriction,  156 

sex,  269 

single,  149 

supplementary,  149,  151,  154 

uniformity,  155 
Genetics,  3 
Genius,  305,  316 
Genotypes,    105,   110,   111,    113, 

116,  196,  250 

Genotypic  selection,  200,  213 
Gemmules,  11,  73 
Germ-cells,  221 

Germplasm,  12,  14,  16,  72,  113, 
196,  254,  315 

architecture  of,  233 

continuity  of,  13,  14,  251 

conservation  of,  330 


346 


INDEX 


Gtermplasm,  desirable,  330 

undesirable,  320 

theory,  84 
GEROTJLD,  51 
GODDARD,  302,  311 
GOLDSCHMIDT,  25,  26,  201,  292 
GOODALE,  289 
Goose,  60 
GOULD,  293,  294 
Graduated  variants,  143 
Grains,  205 
Grasses,  205 
Greeks,  208 
Greening  apples,  11 
Green  peas,  100,  107 
GREGORY,  59,  136,  249 
GRIFFON,  76 
GROUCHY,  263 
Grouse  locust,  249 
Growth,  10 
GRUBER,  217 

Guinea-pigs,  71,  85,  86,  102, 
115,  118,  154,  155,  161,  172, 
233 

GUYER,  89 
Gynandromorphs,  51,  290,  291 

HAECKER,  102,  218 
Haemophilia,  307 
Hair,  305 

angora,  102 

color,  20,  111,  170 

curly,  111 

form,  111 
Hairlessness,  50 
HALDANE,  245 
HALLET,  201 
HANEL,  137 

Hare,  Belgian,  177,  187 
Harelip,  197 
HARTSOEKER,  252 
HATAI,  49 
HAYES,  48,  55,  214 
HAYS,  205 
Hedgehog,  198 
HEGNER,  137 
Helix,  171 
HEIKE,  334 
Helianthus,  48 
Hemiptera,  269 


Hen,  137,  145 
Hereditary  bridge,  228 

character,  8 

defects,  306 

factors,  254 

tunnel,  250 

unit,  148 

Heredity,  definition  of,  6 
Hereford  cattle,  49 
Heritage,  4,  6 
Hermaphroditism,  292 
Hernia,  cerebral,  50 
Heterogametic  female,  270,  271 

male,  267 
Heterosis,  214 
Heterozygote,  105 
Hill  Folk,  303 
Hindrances,  social,  332 
HODGE,  79 
Homozygote,  105 
Homozygous   crosses,   132,   133, 

134,  136,  142 
Honey-bee,  275 
Hooded  rat,  137,  145 
HOOKE,  216 

Hormones,  89,  287,  288 
HORNADAY,  207 
Hornless  cattle,  49,  172 
Hornlessness,  102 
Horns,  102,  171,  172 
Horses,  75,  102,  323 

hairless,  50 

pacing,  102 

trotting,  102 
Hue,  204 
Human  assets,  315 

betterment,  314 

conservation,  314 

egg,   228 

liabilities,  315 

skin  color,   190 

srferm,   228,  252 

stature,   121 

traits,  302 
HURST,  100,  106 
HUXLEY,  336 
Hyalodaphnia,  137 
Hybridization,  200,  209 
Hydra,  137 
Hymenoptera,  275,  276,  277 


INDEX 


347 


IBSEN,  164,  249 
Identical  twins,  18,  142 
Identification,  personal,  19 
Idiots,  322,  325,  329 
Illegitimacy,  325 
Imbecility,  308,  329 
Immigration,  322 
Immortality,  13 
Immunity,  82 

to  rust,  102,  296 
Imperfect   dominance,   168 
Impressions,  maternal,  83 
Inachus,  290 

Inbreeding,  54,  200,  205,  308 
Incomplete  dominance,  174 
Inconstant  species,  46 
Independent  assortment,  103 

unit  characters,  117 
Indians,  208 
Induction,  parallel,  70,  80,  91 

somatic,  70,  91 
Inebriate,  317,  321,  325 
Inheritance,  6 

alternative,  94 

blending,  93,  94 

biological,  7,  63 

criss-cross,  279,  280,  281 

cytoplasmic,  257 

particulate,  94,  155 

sex-limited,  278 

sex-linked,  277,  283,  284 
Inhibitor,  175 
Insane,  305,  317,  320,  321,  322, 

325 

Insanity,  308 
Instinct,  79 
Intensifying  genes,  156 
Interference,  243,  244 
Intergrades,  sex,  290,  291,  292 
Intracellular  pangenesis,  257 
Inventive  genius,  305 
I^otnoea,  205 
Ishmaels,  303 

JANSSENS,  243 

Japanese  art,  19 

JENNINGS,    29,    30,    87,    88,    93, 

123,  136,  138,  139,  140,  141, 

241 
Jews,  208 


Jimson  weed,  59,  174 

JOHANNSEN,  24,  105,  122,  123, 
124,  125,  127,  128,  129,  130, 
132,  135,  141,  146,  149,  203 

JOHNSON,  83 

JOLLOS,  136 

JONES,  249 

JORDAN,  208,  328,  329,  332, 

Jukes,  299,  304 

Juglans,  51 

Jungle  fowl,  60,  196 

Kallikak,  302 
KAMMEEER,  77 
KEEBLE,  48 
KELLICOTT,  296,  327 
KELLY,  137,  249 
KELVIN,  8 

Kernel,  mealiness  of,  136 
KING,  206 
KLEBS,  35,  36 
KNIGHT,  209 

KOELREUTER,    209 

KGRNHATTSER,  290,  292 
KREUGER,  293 

LAMARCK,  34,  42,  64,  65 

Lamb,  legless,  20 

LANG,  102,  171,  174,  179,  189 

LASHLEY,  137 

Lathyrus,  151 

LAUGHLIN,  315,  316,  317,  318, 
330 

Law,  Mendel's,  97,  100,  101,  102, 

117,  168,  211 

of  regression,  121,  122,  130 
of  segregation,  103,  187 

Leaves,  beech,  25,  31,  33 
serrated,  102,  172 
smooth-margined,  102 

LE  COUTOUR,  204 

LEHMANN,  55 

Lemna,  136 

Lentils,  136 

Lepidoptera,  270,  283 

Leprous,  321 

Leptinotarsa,  143,  144 

Lethal  genes,  149,  151,  162,  165 

Liability  to  disease,  312,  317 

Light  reactions,  137 

LILLIE,  288 


348 


INDEX 


LlNDSTROM,    165,    249 

Linkage,  234,  236,  239,  249 

Linnaean  species,  1 

Lint,  102 

Literary  ability,  305 

Live-for-ever,  35,  36 

LITTLE,  162 

Localization  of  genes,  246 

LOCK,  100 

LOEB,  227 

LOTSY,  46 

Lupines,  136 

LYELL,  167 

MACDONALD,  317 
MACDOUGAL,    46 

MACDOWELL,  137,  145 

Maize,   102,   137,  165,  186,  207, 

214,  249 
Mammals,  288 
Man,  102 

primitive,  296 
Mangro,  142 
Mantle  fibers,  220 
Marriage,  consanguineous,  208, 

312 

cousin,  209,  307 
laws,  324 
taboo,  325 
Mass  selection,  200 
Maternal  impressions,  83 
Mathematical  aptitude,  305 
Maturation,   53,    113,   223,   224, 

230,  268 
MAY,  53,  137 
McCLUNG,  265 
MEADER,  136 
Mealiness  of  kernel,  136 
Mean,  arithmetical,  23,  26,  42 
Meiosis,  54 
Melanism,  199 
Melting-pot,  93,  176,  186 
Membrane,  nuclear,  216 
MENDEL,  3,  23,  95,  97,  98,  102, 
106,  107,  110,  114,  119,  148, 
168,  173,  211,  234,  235,  249, 
317 
Mendel's  law,  97,  100,  101,  102, 

117,  168,  211,  297 
Mendelism,  93,  250,  254 


MENDIOLA,  136 

Mental  characteristics,  303 

Merino  sheep,  49 

Metaclonosis,  55 

Metaphase,  220,  267,  271,  273 

METZ,  52,  249 

Mice,  75,  101,  102,  161,  206,  249 

hairless,  50 

in  abnormal  temperature,  76 

intensified,  156 

piebald,  95,  155 

ricin-immune,  82 

spotted,  52,  155 

waltzing,  102 

yellow,  162 
Micron,  139 
MIDDLE-TOST,  136 
MILTON,  9 
Mirabilis,  106,  174 
Mitosis,  219 
Mode,  26,  27 
Modifications,  38,  61 
Modifying  genes.  ^46,  149,  150, 

166 

Mohammedans,  208 
Mold,  83 
Mollusks,  293 
Monohybrid,  99,  103,  19tfL 

MONTGQMERY,   67,  69 

Moral  characters,  303 

MORGAN,  LLOYD,  73 

MORGAN,  T.  H.,  53,  97,  102,  103, 
132,  165,  170,  171,  234,  235, 
236,  237,  239,  240,  242,  243, 
245,  246,  272,  285,  287,  289, 
291 

Morning-glories,  205 

Mosaics,  292 

Mcths,  70,  270,  272,  284,  292 

Mudpuppy,  79 

Mulatto,  190,  191,  192 

Mule,  214 

MULLENIX,  162,  177,  178 

MULLER,  54,  244,  246 

Multiple  mutation,  51 
variation,  20 

Museum  of  heredity,  132 

Musical  ability,  305 

Mustifee,  192 

Mustifino,  192 


INDEX 


349 


Mutation,  23,  38,  39,  40,  51,  55, 
61,  62,  91,  254 

aggregate,  51 

causes  of,  57,  60 

factor,  58,  60 

gametic,  53 

kinds  of,  51 

multiple,  51 

origin  of,  53 

parallel,  52 

recurrent,  53 

reverse,  53 

single  gene,  51 

somatic,  53,  55 

theory,  41 

zygotic,  53,  55 
Mutilation,  75 

NABOURS,  279 

NACHTSHEIM,  276 

NAEGELI,  20,  21,  95 

Nams,  303 

Narcissus,  59 

Natural    selection,    41,    42,    51, 

297 

Nature,  63,  203 
Necturus,  79 
Negro,  192 

Negro-white  cross,  190 
Nematodes,  293 
Neo-Darwinian,  65 
Neo-Lamarckian,  65 
Neotony,  78 
Nettles,  102,  172 
NEWELL,  276 
NEWTON,  316,  334 
Nicodemus,  72 
Night-blindness,  312 
NILSSON,  136,  205 
NILSSON-EHLE,    179,    180,    181, 

183,  186,  187,  191 

NlTOBE,  19 

Non-disjunction,   59,    269,   285, 

286,  287 

Nuclear  membrane,  216 
Nucleus,  916 
Nulliplex,  107,  195 
Nurture  63,  203 

Oats,  136,  249 
Octaroon,  192 


(Enothera,  43,  44,  45,  59,  165 

Oil-gland,  50 

Oocyte,  224,  267,  271,  273 

Oogonia,  224,  267,  271,  273 

Oranges,  navel,  11 

Origin  of  individual,  2 

life,  8 

mutation,  53 

species,  3,  41 
Orthoptera,  265 
OSBORN,  336 
Ostrich,  52 

Outcrossing,  207,  209,  307 
Ovarian  transplantation,  85,  86 
OVID,  9 
Ovists,  222 

Ovum,  mature,  267,  271,  273 
Oyster-borer,  28,  29 

Pacing  horses,  102 

Pangenesis,  70,  73,  90,  91,  105 
intracellular,  257 

Palatine  ridges,  197 

Papaver,  47 

Parallel  induction,  70,  81,  91 
mutation,  52 

Paramerium,  29,  30,  87,  88,  136, 
137,  138,  140,  141 

Parasitism,  290 

Parental   generation,   103 

Parrot,  20 

Participate  inheritance,  94,  155 

Parthenogenesis,  132,  134,  227 

Parthenogenetic  eggs,  273,  275 
progeny,  132,  134,  136,  141 

Partial  potency,  174 

PASTETJR,  9,  82,  316 

Pattern  genes,  154,  161 

Paupers,  299,  305,  309,  317,  322 

PAYNE,  137 

PEARL,  136,  137,  145 

PEARSON*  24,  25,  31 

Peas,  98,  110,  136,  148,  205,  234, 

249 

dwarf,  98,  99,  100,  105,  107 
garden,  100 
green,  100,  107 
smooth,  100,  107,  173 
sweet,  151,  152,  235,  249 
tall,  98,  99,  100,  105,  107 


350 


INDEX 


Peas,  wrinkled,  100, 107, 173 

yellow,  100,  107,  110 
Pebrine,  82 

Pedigree  breeding,  200,  203 
Pendulum,  40 
PenicilUum,  83 
Peromyscus,  52 
Persians,  208 
PETRUNKEVITSCH,  276 
Petunia,  47 
Phacechaerus,  78 
Phaseolus,  123 
Phenotype,   110,   111,  113,  116, 

159,  196,  250 
Phenotypic  selection,  201 
PHILLIPS,  85,  86,  137 
Phoenicians,  208 
Phylloxera,  272,  273,  275 
Physiological  regulators,  262 
Piebald  mice,  95,  155 
Pied  Piper,  145 
Pigeon,  196,  249 
Pigment  gene,  152 

production,  136 
Pigs,  171 
Pineys,  303 
Plesiosaurs,  56 
Plural  genes,  149 
Pomace  fly  (see  Drosophila) 
Polarity,  259 
Polydactylism,  312 
Polyembryony,  277 
Polygons,  bimodal,  29 

frequency,  29 
Poppy,  Shirley,  47 
Polar   cells,   54,   224,  267,   271, 

273 

POPEXOE,  83 
Population,  127 
Porcupine,  198 

Poultry,    50,    71,    89,    102,    124, 
171,  176 

rumpless,   172,   175 

tailless,   49 
Potato,  136,  205 

beetles,  33,  36,  137,  143,  -144 
Potency,  172 

failure  of,  174 

partial,  174 

total,  173 


Preformation,  252 
Prenatal  characters,  68 

influences,  83 
Presence     or     absence     theory, 

106,  149 

Preventable  death,  321 
PRICE,   102 
Primitive  man,  296 
Primrose,  double,  47 

evening,  249 

giant,  48 
Primula,  59,  249 
Prisoners,  309 
Progeny,   parthenogenetic,    132, 

134,  136,  141 
Progressive  species,  45 
Prophase,  219 
Prostitutes,  299,  322 
Protophyta,  10 
Protoplasm,  216 
Protozoa,  10,  13,  86,  138 
Proximity,  308,  328 
Pseudomethoca,  291 
Pug  jaws,  50 
PUKNETT,  99,  235,  249 
Pure  lines,  121,  122,  123 
Puritan  stock,  208 
Purple  beech,  47 

Quadroon,  192 
Quail,  79 

Rabbit,    16,   89,    101,   162,   163. 
249,  268 

color  of,  160 

ears,  177,  187 

gray,  162 

lenses,  89 

lop-eared,  177 

phenotypes,  159 
Race  preservation,  336 
Radiolaria,  218 
Rate  of  development,  262 
Rats,  149,  206,  249 

hooded,  139,  145,  166 
Recessive,  99,  119,  196,  212 

extracted,   310 
Recurrent  mutation,  53 
Red-eyed  yellow  mice,  52 
REDFIELD,  67 


INDEX 


351 


Reduction  division,  58,  59 

REEVES,  137 

Regression,   121,   122,   125,   130, 

199 

Regulators,  physiological,  262 
REID,  66 
Reinfection,  82 
Relative  variability,  28 
Repair,  9 

Reproduction,  sexual,   12,  221 
Reproductive  period,  56 
Resistance  to  poisoning,  82 
Response,  4,  6 
Restriction  gene,  157 
Retrogressive  varieties,  45 
Reversed  dominance,  171 
Reverse  mutation,  53 
Reversion,  131,  151,  194 

explanation  of,  199 

false,  197 
Rhabdites,  293 
RHODES,  316 
Rice,  204 

Ricin  immunity,  82 
RIDDLE,  71,  272,  295 
RIMPAU,  202 
Ringer's  solution,  89 
RITZEMA-BOS,  206 
Roan  color,  170 
Rodents,  171 
Romans,  208 
ROOT,  137 
Roses,  47 
Rotifers,  274,  275 
Round  worms,  293 
Rudbeckia,  174 
Rudimentary  wing,  53 
Rumplessness,  102,  172,  175 
Rust,  immunity  to,  102,  296 

Sacculina,  290 
Salamander,  77,  102 
Salamandra,  77 
Sambo,  192 
SA  TINDERS,  102 
Scale  of  success,  6 
Scalp  muscles,  197 

SCHAFFNER,  48 
SCHLEIDEN,  215 
SCHWANN,  215 


Sea-urchin,  230,  231 
Secondary     sexual     characters, 

287 

Sedum,  35,  36 
Segregation,  103,  113,  119,  168 

of  defectives,  327 
SEILER,  270,  272,  275 
Selection,  121,  166 

genotypic,  200,  213 

mass,  200 

natural,  41,  42,  51 

phenotypic,  201 

pure  line,  133,  136,  137 
Self  fertilization,  97,  134 
Sex  cells,  224 

chromosome,  245,  265 

cycle,  272 

determination  of,  264,  265 

genes,  269 

intergrades,  290,  291,  292 

linked,  277,  283,  284 
Sexual  reproduction,  12,  221 
SHAKESPEARE,  63,  316 
SHARP,  242,  247,  248,  285,  287 
Sheep,  18,  48,  75,  171 
Shirley  poppy,  47 
SHIRREFF,  204 
SHULL,  A.  F.,  275, 
SHULL,  G.  S.,  46,  76,  102,  105, 

106,  165,  207,  214,  249 
Siamese  twins,  50 
Silkworms,  82,  102,  249 
Simocephalus,  137 
Simplex,  107,  195 
Single  genes,  149 
SITKOWSKI,  70 
Skew  curves,  31 
Skin  color,  190 
Simplex,  107,  195 
SMITH,  89,  137,  275,  290 
Smooth-margined  leaves,  102 
Smooth  peas,  100,  107,  173 
Snails,  28,  33,  102,  171,  174 
Snapdragon,  102,  165,  249 
Social  hindrances,  332 
Somatic  characters,  67 

differentiation,  255 
Somatic  induction,  70,  91 

mutation,  53,  55 

variation,  21 


352 


INDEX 


Scmatogenesis,  250,  253,  254 
Somatoplasm,   12,    14,    72,    113, 

196,  254 
Soy  beans,  136 
Species,  cycle,  56 

definition  of,  1 

degressive,  46 

elementary,  41 

Linnaean*  1 

inconstant,  46 

origin  of,  3,  41 

progressive,  41,  45 
SPENCER,  79,  334 
Sperm,  human,  228,  252 
Spermatid,  224,  267,  271 
Spermatocyte,     224,    267,    271, 

273 
Spermatogonia,    224,    267,    271, 

273 
Spermatozoa,  221,  224,  226,  267, 

271,  273 
Spermists,  222 
Sphcerechinus,  231 
SPILLMAN,  102 
Spines,  dermal,  199 
Spiny  ant-eater,  198 
Sponges*  10,  11 
Spontaneous  generation,  9 
Spores,  12 

Sports,  20,  40,  42,  199 
Spotted  mice,  52,  155 
SPRENGER,  47 
Spurs,  50 

Standard  deviation,  26,  27 
STANDFUSS,  55 
Starchy  kernel,  102 
Starfish,  22,  25,  26,  33 
Statoblasts,  11 
Stature,  human,  121 
Stentor,  217 
Sterilization,  330 
STEVENSON,  334 
STIEGLEDER,  164 
Stock,  47 
STOCKARD,  20 
STOCKING,  136 
STOMPS,  59 
STOUT,  136 
Struthio,  52 
STURTEVANT,  59,  137,  245,  246 


Styela,  259 

Stylonychia,  136 

Submerged  tenth,  320 

Subsidizing  the  fit,  333 

Sudan   red,   71 

Sugar  beet,  205 

Sugary  kernel,  102 

SUMNER,  52,  76 

Sunburn,  76 

Sunflower,  48,  102 

Supplementary  genes,  149,  151, 

154 

SURFACE,  136,  249 
Survival  of  the  fittest,  298 
Susceptibility    to    disease,    312, 

317 

rust,  102 

Sweet  peas,  151,  152,  235,  249 
Synapsis,  58 
Syndesis,  241 

Taboo,  325 

Tadpoles,  78 

Taillessness,  49 

Tails,  docked,  75 

Tall  peas,  98,  99,  100,  105,  107 

TANAKA,  249 

Tattooing,  75 

Tatusia,  277 

Telophase,  220,  221 

TENNENT,    231 

Tetraploidy,  59 

Tetrads,  266 

Texas  fever,  82 

Thelia,  290,  292 

Theromorphs,  56 

Theory,  cell,  215 

chiasmatype,   243 

chromosome,  229,  254 

mutation,  41 

pre formation,    253 
Thigh-bones,  absent,  50 
THOMSON,  194 
Thoracic     bristles,     137,      143, 

145 

Thumb  prints,  19 
Tineola,  70 
Tobacco,  48,  52,  214 
Toenails,  absent,  50 

extra,  50 


INDEX 


353 


Toes,  crippled,  75 

extra,  50,  171 

reduced,  172 

webbed,   50 

Tomato,  52,  59,  102,  249 
Total  potency,  173 
TOWER,  33,  36,  137,  143,  144 
TOYAMA,  102 
Training,  5 
Traits,  human,  304 
Transmission  of  disease,  80 
Transplantation  of  ovaries,  85, 

86,  289 
Treasury  of  human  inheritance, 

318 

Trees  deformed  by  wind,  76 
Triangle  of  life,  3,  4 
Trihybrid,  114,  183,  185 
Trilobites,  56 
Trotting  horses,  102 
TSCHERMAK,  96,  100,  102 
Tuberculosis,    20,    68,    80,    311, 

321,  322 
Twins,  identical,  18,  142 

Siamese,  50 
TYNDALL,  9 

Unhanded  shell,  102 
Unbranched  habit,  102 
Undesirable  germplasm,  320 
Uniformity  gene,  155 
Unit  characters,  2 
hereditary,    148 
Urosalpinx,  28,  29 
Use,  effects  of,  78 

VAN  BENEDEN,  221,  223 
Variability,  coefficient  of,  26 

relative,  28 

Variants,  graduated,  143 
Variation,  17,  254 

abnormal,  22 

bud,  55 

causes  of,  34 

continuous,  22 

convergent,  198 

definite,  21 

discontinuous,  22 

fluctuating,  23,  24,  42,  51,  62, 
199 


Variation,  fortuitous,  21 

germinal,  22 

graduated,  34 

harmful,  21 

hereditary,  23,  61 

indefinite,  21 

integral,  33 

indifferent,  20 

kinds  of,  19 

morphological,  19 

multiple,  20 

non-hereditary,   23 

normal,  22 

orthogenetic,  21 

physiological,  19 

psychological,  20 

qualitative,  23 

quantitative,  22 

single,  20 

somatic,  21 

useful,  20 

universality  of,  18 
Varieties,  regressive,  48 

retrogressive,  45 
Variety,  38 
Vasectomy,  329 
Venereal  diseases,  325 
Verbena,  48 

Vermiform  appendix,  21,  197 
Vestigial  structures,  197 
VILMORIN,  135,  136,  205 
Viola,  18 
VOGLER,  136 
Volta  Bureau,  318 
VOLTAIRE,  334 
VON  BAEHR,  272 
VON  KOELLIKER,  221 
VON  MOHL,  216 

Walnut,  51 

WALKER,  136 

WALTER,  29,  162,  177,  178 

Waltzing  mice,  102 

War,  331,  332 

Wart-hog,  78 

Wasp,  291 

Waterloo,  263 

Weakling  babies,  334 

Webbed  toes,  50 

WEBBER,  132 


354 


INDEX 


WEISMANN,  10,  35,  65,  67,  69, 
71,  73,  74,  75,  84,  88,  206, 
233,  255,  256 

Wheat,  102,  135,  136,  180,  191, 
201,  204,  249,  296 

WHITE,  100,  249 

WHITNEY,  275 

WHYMPER,  329 

WlEDERSHEIM,    75 


William  the  Conqueror,  208 
Willows,  76 
WILSON,  232 
Wings,  absent,  50 

cut,  53 

rudimentary,  53 
WINKLEB,  59 
WINSHIP,  301 
WINSLOW,  136 
Winter  eggs,  273,  274 
WOLF,  136 
WOLFF,  252 
WOLTZBZCK,  34,  35,  137 


WOODS,  304 
Worms,  flat,  293 

nematode,  14,  218 

round,  293 

silk,  82,  102,  249 
WRIGHT,  48 
Wrinkled  peas,  100,  107,  173 

X-chromosome,    266,    268,    272, 
277,  280,  285 

Y-chromosome,  268-279 
Yellow  mice,  162 
Yield  per  acre,  136 
Youth,  56 

Z-chromosome,  283,  284 
ZEDERBAUR,  77 
ZELENY,  137 
Zeros,  303 

ZlEGI-ER,  47 

Zygote,  223 

Zygotic  mutation,  53,  55 


354 


INDEX 


WEISMANN,  10,  35,  65,  67,  69, 
71,  73,  74,  75,  84,  88,  206, 
233,  255,  256 

Wheat,  102,  135,  136,  180,  191, 
201,  204,  249,  296 

WHITE,  100,  249 

WHITNEY,  275 

WHYMPER,  329 

WlEDERSHEIM,    75 
WlLKS,  47 

William  the  Conqueror,  208 
Willows,  76 
WILSON,  232 
Wings,  absent,  50 

cut,  53 

rudimentary,  53 
WINKLER,  59 
WINSHIP,  301 
WINSLOW,  136 
Winter  eggs,  273,  274 
WOLF,  136 
WOLFF,  252 
WOLTEHECK,  34,  35,  137 


WOODS,  304 
Worms,  flat,  293 

nematode,  14,  218 

round,  293 

silk,  82,  102,  249 
WRIGHT,  48 
Wrinkled  peas,  100,  107,  173 

X-chromosome,    266,    268,    272, 
277,  280,  285 

Y-chromosome,   268-279 
Yellow  mice,  162 
Yield  per  acre,  136 
Youth,  56 

Z-chromosome,  283,  284 
ZEDERBAUR,  77 
ZELENY,  137 
Zeros,  303 

ZlEGLER,   47 

Zygote,  223 

Zygotic  mutation,  53,  55 


^^^ 


BIOLOGY 
LIBRARY 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


